relocation.c

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/*
 * Copyright (C) 2009 Oracle.  All rights reserved.
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public
 * License v2 as published by the Free Software Foundation.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * General Public License for more details.
 *
 * You should have received a copy of the GNU General Public
 * License along with this program; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA 021110-1307, USA.
 */

#include <linux/sched.h>
#include <linux/pagemap.h>
#include <linux/writeback.h>
#include <linux/blkdev.h>
#include <linux/rbtree.h>
#include <linux/slab.h>
#include "ctree.h"
#include "disk-io.h"
#include "transaction.h"
#include "volumes.h"
#include "locking.h"
#include "btrfs_inode.h"
#include "async-thread.h"
#include "free-space-cache.h"
#include "inode-map.h"

/*
 * backref_node, mapping_node and tree_block start with this
 */
struct tree_entry {
    struct rb_node rb_node;
    u64 bytenr;
};

/*
 * present a tree block in the backref cache
 */
struct backref_node {
    struct rb_node rb_node;
    u64 bytenr;

    u64 new_bytenr;
    /* objectid of tree block owner, can be not uptodate */
    u64 owner;
    /* link to pending, changed or detached list */
    struct list_head list;
    /* list of upper level blocks reference this block */
    struct list_head upper;
    /* list of child blocks in the cache */
    struct list_head lower;
    /* NULL if this node is not tree root */
    struct btrfs_root *root;
    /* extent buffer got by COW the block */
    struct extent_buffer *eb;
    /* level of tree block */
    unsigned int level:8;
    /* is the block in non-reference counted tree */
    unsigned int cowonly:1;
    /* 1 if no child node in the cache */
    unsigned int lowest:1;
    /* is the extent buffer locked */
    unsigned int locked:1;
    /* has the block been processed */
    unsigned int processed:1;
    /* have backrefs of this block been checked */
    unsigned int checked:1;
    /*
     * 1 if corresponding block has been cowed but some upper
     * level block pointers may not point to the new location
     */
    unsigned int pending:1;
    /*
     * 1 if the backref node isn't connected to any other
     * backref node.
     */
    unsigned int detached:1;
};

/*
 * present a block pointer in the backref cache
 */
struct backref_edge {
    struct list_head list[2];
    struct backref_node *node[2];
};

#define LOWER   0
#define UPPER   1

struct backref_cache {
    /* red black tree of all backref nodes in the cache */
    struct rb_root rb_root;
    /* for passing backref nodes to btrfs_reloc_cow_block */
    struct backref_node *path[BTRFS_MAX_LEVEL];
    /*
     * list of blocks that have been cowed but some block
     * pointers in upper level blocks may not reflect the
     * new location
     */
    struct list_head pending[BTRFS_MAX_LEVEL];
    /* list of backref nodes with no child node */
    struct list_head leaves;
    /* list of blocks that have been cowed in current transaction */
    struct list_head changed;
    /* list of detached backref node. */
    struct list_head detached;

    u64 last_trans;

    int nr_nodes;
    int nr_edges;
};

/*
 * map address of tree root to tree
 */
struct mapping_node {
    struct rb_node rb_node;
    u64 bytenr;
    void *data;
};

struct mapping_tree {
    struct rb_root rb_root;
    spinlock_t lock;
};

/*
 * present a tree block to process
 */
struct tree_block {
    struct rb_node rb_node;
    u64 bytenr;
    struct btrfs_key key;
    unsigned int level:8;
    unsigned int key_ready:1;
};

#define MAX_EXTENTS 128

struct file_extent_cluster {
    u64 start;
    u64 end;
    u64 boundary[MAX_EXTENTS];
    unsigned int nr;
};

struct reloc_control {
    /* block group to relocate */
    struct btrfs_block_group_cache *block_group;
    /* extent tree */
    struct btrfs_root *extent_root;
    /* inode for moving data */
    struct inode *data_inode;

    struct btrfs_block_rsv *block_rsv;

    struct backref_cache backref_cache;

    struct file_extent_cluster cluster;
    /* tree blocks have been processed */
    struct extent_io_tree processed_blocks;
    /* map start of tree root to corresponding reloc tree */
    struct mapping_tree reloc_root_tree;
    /* list of reloc trees */
    struct list_head reloc_roots;
    /* size of metadata reservation for merging reloc trees */
    u64 merging_rsv_size;
    /* size of relocated tree nodes */
    u64 nodes_relocated;

    u64 search_start;
    u64 extents_found;

    unsigned int stage:8;
    unsigned int create_reloc_tree:1;
    unsigned int merge_reloc_tree:1;
    unsigned int found_file_extent:1;
    unsigned int commit_transaction:1;
};

/* stages of data relocation */
#define MOVE_DATA_EXTENTS   0
#define UPDATE_DATA_PTRS    1

static void remove_backref_node(struct backref_cache *cache,
                struct backref_node *node);
static void __mark_block_processed(struct reloc_control *rc,
                   struct backref_node *node);

static void mapping_tree_init(struct mapping_tree *tree)
{
    tree->rb_root = RB_ROOT;
    spin_lock_init(&tree->lock);
}

static void backref_cache_init(struct backref_cache *cache)
{
    int i;
    cache->rb_root = RB_ROOT;
    for (i = 0; i < BTRFS_MAX_LEVEL; i++)
        INIT_LIST_HEAD(&cache->pending[i]);
    INIT_LIST_HEAD(&cache->changed);
    INIT_LIST_HEAD(&cache->detached);
    INIT_LIST_HEAD(&cache->leaves);
}

static void backref_cache_cleanup(struct backref_cache *cache)
{
    struct backref_node *node;
    int i;

    while (!list_empty(&cache->detached)) {
        node = list_entry(cache->detached.next,
                  struct backref_node, list);
        remove_backref_node(cache, node);
    }

    while (!list_empty(&cache->leaves)) {
        node = list_entry(cache->leaves.next,
                  struct backref_node, lower);
        remove_backref_node(cache, node);
    }

    cache->last_trans = 0;

    for (i = 0; i < BTRFS_MAX_LEVEL; i++)
        BUG_ON(!list_empty(&cache->pending[i]));
    BUG_ON(!list_empty(&cache->changed));
    BUG_ON(!list_empty(&cache->detached));
    BUG_ON(!RB_EMPTY_ROOT(&cache->rb_root));
    BUG_ON(cache->nr_nodes);
    BUG_ON(cache->nr_edges);
}

static struct backref_node *alloc_backref_node(struct backref_cache *cache)
{
    struct backref_node *node;

    node = kzalloc(sizeof(*node), GFP_NOFS);
    if (node) {
        INIT_LIST_HEAD(&node->list);
        INIT_LIST_HEAD(&node->upper);
        INIT_LIST_HEAD(&node->lower);
        RB_CLEAR_NODE(&node->rb_node);
        cache->nr_nodes++;
    }
    return node;
}

static void free_backref_node(struct backref_cache *cache,
                  struct backref_node *node)
{
    if (node) {
        cache->nr_nodes--;
        kfree(node);
    }
}

static struct backref_edge *alloc_backref_edge(struct backref_cache *cache)
{
    struct backref_edge *edge;

    edge = kzalloc(sizeof(*edge), GFP_NOFS);
    if (edge)
        cache->nr_edges++;
    return edge;
}

static void free_backref_edge(struct backref_cache *cache,
                  struct backref_edge *edge)
{
    if (edge) {
        cache->nr_edges--;
        kfree(edge);
    }
}

static struct rb_node *tree_insert(struct rb_root *root, u64 bytenr,
                   struct rb_node *node)
{
    struct rb_node **p = &root->rb_node;
    struct rb_node *parent = NULL;
    struct tree_entry *entry;

    while (*p) {
        parent = *p;
        entry = rb_entry(parent, struct tree_entry, rb_node);

        if (bytenr < entry->bytenr)
            p = &(*p)->rb_left;
        else if (bytenr > entry->bytenr)
            p = &(*p)->rb_right;
        else
            return parent;
    }

    rb_link_node(node, parent, p);
    rb_insert_color(node, root);
    return NULL;
}

static struct rb_node *tree_search(struct rb_root *root, u64 bytenr)
{
    struct rb_node *n = root->rb_node;
    struct tree_entry *entry;

    while (n) {
        entry = rb_entry(n, struct tree_entry, rb_node);

        if (bytenr < entry->bytenr)
            n = n->rb_left;
        else if (bytenr > entry->bytenr)
            n = n->rb_right;
        else
            return n;
    }
    return NULL;
}

void backref_tree_panic(struct rb_node *rb_node, int errno,
                      u64 bytenr)
{

    struct btrfs_fs_info *fs_info = NULL;
    struct backref_node *bnode = rb_entry(rb_node, struct backref_node,
                          rb_node);
    if (bnode->root)
        fs_info = bnode->root->fs_info;
    btrfs_panic(fs_info, errno, "Inconsistency in backref cache "
            "found at offset %llu\n", (unsigned long long)bytenr);
}

/*
 * walk up backref nodes until reach node presents tree root
 */
static struct backref_node *walk_up_backref(struct backref_node *node,
                        struct backref_edge *edges[],
                        int *index)
{
    struct backref_edge *edge;
    int idx = *index;

    while (!list_empty(&node->upper)) {
        edge = list_entry(node->upper.next,
                  struct backref_edge, list[LOWER]);
        edges[idx++] = edge;
        node = edge->node[UPPER];
    }
    BUG_ON(node->detached);
    *index = idx;
    return node;
}

/*
 * walk down backref nodes to find start of next reference path
 */
static struct backref_node *walk_down_backref(struct backref_edge *edges[],
                          int *index)
{
    struct backref_edge *edge;
    struct backref_node *lower;
    int idx = *index;

    while (idx > 0) {
        edge = edges[idx - 1];
        lower = edge->node[LOWER];
        if (list_is_last(&edge->list[LOWER], &lower->upper)) {
            idx--;
            continue;
        }
        edge = list_entry(edge->list[LOWER].next,
                  struct backref_edge, list[LOWER]);
        edges[idx - 1] = edge;
        *index = idx;
        return edge->node[UPPER];
    }
    *index = 0;
    return NULL;
}

static void unlock_node_buffer(struct backref_node *node)
{
    if (node->locked) {
        btrfs_tree_unlock(node->eb);
        node->locked = 0;
    }
}

static void drop_node_buffer(struct backref_node *node)
{
    if (node->eb) {
        unlock_node_buffer(node);
        free_extent_buffer(node->eb);
        node->eb = NULL;
    }
}

static void drop_backref_node(struct backref_cache *tree,
                  struct backref_node *node)
{
    BUG_ON(!list_empty(&node->upper));

    drop_node_buffer(node);
    list_del(&node->list);
    list_del(&node->lower);
    if (!RB_EMPTY_NODE(&node->rb_node))
        rb_erase(&node->rb_node, &tree->rb_root);
    free_backref_node(tree, node);
}

/*
 * remove a backref node from the backref cache
 */
static void remove_backref_node(struct backref_cache *cache,
                struct backref_node *node)
{
    struct backref_node *upper;
    struct backref_edge *edge;

    if (!node)
        return;

    BUG_ON(!node->lowest && !node->detached);
    while (!list_empty(&node->upper)) {
        edge = list_entry(node->upper.next, struct backref_edge,
                  list[LOWER]);
        upper = edge->node[UPPER];
        list_del(&edge->list[LOWER]);
        list_del(&edge->list[UPPER]);
        free_backref_edge(cache, edge);

        if (RB_EMPTY_NODE(&upper->rb_node)) {
            BUG_ON(!list_empty(&node->upper));
            drop_backref_node(cache, node);
            node = upper;
            node->lowest = 1;
            continue;
        }
        /*
         * add the node to leaf node list if no other
         * child block cached.
         */
        if (list_empty(&upper->lower)) {
            list_add_tail(&upper->lower, &cache->leaves);
            upper->lowest = 1;
        }
    }

    drop_backref_node(cache, node);
}

static void update_backref_node(struct backref_cache *cache,
                struct backref_node *node, u64 bytenr)
{
    struct rb_node *rb_node;
    rb_erase(&node->rb_node, &cache->rb_root);
    node->bytenr = bytenr;
    rb_node = tree_insert(&cache->rb_root, node->bytenr, &node->rb_node);
    if (rb_node)
        backref_tree_panic(rb_node, -EEXIST, bytenr);
}

/*
 * update backref cache after a transaction commit
 */
static int update_backref_cache(struct btrfs_trans_handle *trans,
                struct backref_cache *cache)
{
    struct backref_node *node;
    int level = 0;

    if (cache->last_trans == 0) {
        cache->last_trans = trans->transid;
        return 0;
    }

    if (cache->last_trans == trans->transid)
        return 0;

    /*
     * detached nodes are used to avoid unnecessary backref
     * lookup. transaction commit changes the extent tree.
     * so the detached nodes are no longer useful.
     */
    while (!list_empty(&cache->detached)) {
        node = list_entry(cache->detached.next,
                  struct backref_node, list);
        remove_backref_node(cache, node);
    }

    while (!list_empty(&cache->changed)) {
        node = list_entry(cache->changed.next,
                  struct backref_node, list);
        list_del_init(&node->list);
        BUG_ON(node->pending);
        update_backref_node(cache, node, node->new_bytenr);
    }

    /*
     * some nodes can be left in the pending list if there were
     * errors during processing the pending nodes.
     */
    for (level = 0; level < BTRFS_MAX_LEVEL; level++) {
        list_for_each_entry(node, &cache->pending[level], list) {
            BUG_ON(!node->pending);
            if (node->bytenr == node->new_bytenr)
                continue;
            update_backref_node(cache, node, node->new_bytenr);
        }
    }

    cache->last_trans = 0;
    return 1;
}


static int should_ignore_root(struct btrfs_root *root)
{
    struct btrfs_root *reloc_root;

    if (!root->ref_cows)
        return 0;

    reloc_root = root->reloc_root;
    if (!reloc_root)
        return 0;

    if (btrfs_root_last_snapshot(&reloc_root->root_item) ==
        root->fs_info->running_transaction->transid - 1)
        return 0;
    /*
     * if there is reloc tree and it was created in previous
     * transaction backref lookup can find the reloc tree,
     * so backref node for the fs tree root is useless for
     * relocation.
     */
    return 1;
}
/*
 * find reloc tree by address of tree root
 */
static struct btrfs_root *find_reloc_root(struct reloc_control *rc,
                      u64 bytenr)
{
    struct rb_node *rb_node;
    struct mapping_node *node;
    struct btrfs_root *root = NULL;

    spin_lock(&rc->reloc_root_tree.lock);
    rb_node = tree_search(&rc->reloc_root_tree.rb_root, bytenr);
    if (rb_node) {
        node = rb_entry(rb_node, struct mapping_node, rb_node);
        root = (struct btrfs_root *)node->data;
    }
    spin_unlock(&rc->reloc_root_tree.lock);
    return root;
}

static int is_cowonly_root(u64 root_objectid)
{
    if (root_objectid == BTRFS_ROOT_TREE_OBJECTID ||
        root_objectid == BTRFS_EXTENT_TREE_OBJECTID ||
        root_objectid == BTRFS_CHUNK_TREE_OBJECTID ||
        root_objectid == BTRFS_DEV_TREE_OBJECTID ||
        root_objectid == BTRFS_TREE_LOG_OBJECTID ||
        root_objectid == BTRFS_CSUM_TREE_OBJECTID)
        return 1;
    return 0;
}

static struct btrfs_root *read_fs_root(struct btrfs_fs_info *fs_info,
                    u64 root_objectid)
{
    struct btrfs_key key;

    key.objectid = root_objectid;
    key.type = BTRFS_ROOT_ITEM_KEY;
    if (is_cowonly_root(root_objectid))
        key.offset = 0;
    else
        key.offset = (u64)-1;

    return btrfs_read_fs_root_no_name(fs_info, &key);
}

#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
static noinline_for_stack
struct btrfs_root *find_tree_root(struct reloc_control *rc,
                  struct extent_buffer *leaf,
                  struct btrfs_extent_ref_v0 *ref0)
{
    struct btrfs_root *root;
    u64 root_objectid = btrfs_ref_root_v0(leaf, ref0);
    u64 generation = btrfs_ref_generation_v0(leaf, ref0);

    BUG_ON(root_objectid == BTRFS_TREE_RELOC_OBJECTID);

    root = read_fs_root(rc->extent_root->fs_info, root_objectid);
    BUG_ON(IS_ERR(root));

    if (root->ref_cows &&
        generation != btrfs_root_generation(&root->root_item))
        return NULL;

    return root;
}
#endif

static noinline_for_stack
int find_inline_backref(struct extent_buffer *leaf, int slot,
            unsigned long *ptr, unsigned long *end)
{
    struct btrfs_extent_item *ei;
    struct btrfs_tree_block_info *bi;
    u32 item_size;

    item_size = btrfs_item_size_nr(leaf, slot);
#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
    if (item_size < sizeof(*ei)) {
        WARN_ON(item_size != sizeof(struct btrfs_extent_item_v0));
        return 1;
    }
#endif
    ei = btrfs_item_ptr(leaf, slot, struct btrfs_extent_item);
    WARN_ON(!(btrfs_extent_flags(leaf, ei) &
          BTRFS_EXTENT_FLAG_TREE_BLOCK));

    if (item_size <= sizeof(*ei) + sizeof(*bi)) {
        WARN_ON(item_size < sizeof(*ei) + sizeof(*bi));
        return 1;
    }

    bi = (struct btrfs_tree_block_info *)(ei + 1);
    *ptr = (unsigned long)(bi + 1);
    *end = (unsigned long)ei + item_size;
    return 0;
}

/*
 * build backref tree for a given tree block. root of the backref tree
 * corresponds the tree block, leaves of the backref tree correspond
 * roots of b-trees that reference the tree block.
 *
 * the basic idea of this function is check backrefs of a given block
 * to find upper level blocks that refernece the block, and then check
 * bakcrefs of these upper level blocks recursively. the recursion stop
 * when tree root is reached or backrefs for the block is cached.
 *
 * NOTE: if we find backrefs for a block are cached, we know backrefs
 * for all upper level blocks that directly/indirectly reference the
 * block are also cached.
 */
static noinline_for_stack
struct backref_node *build_backref_tree(struct reloc_control *rc,
                    struct btrfs_key *node_key,
                    int level, u64 bytenr)
{
    struct backref_cache *cache = &rc->backref_cache;
    struct btrfs_path *path1;
    struct btrfs_path *path2;
    struct extent_buffer *eb;
    struct btrfs_root *root;
    struct backref_node *cur;
    struct backref_node *upper;
    struct backref_node *lower;
    struct backref_node *node = NULL;
    struct backref_node *exist = NULL;
    struct backref_edge *edge;
    struct rb_node *rb_node;
    struct btrfs_key key;
    unsigned long end;
    unsigned long ptr;
    LIST_HEAD(list);
    LIST_HEAD(useless);
    int cowonly;
    int ret;
    int err = 0;

    path1 = btrfs_alloc_path();
    path2 = btrfs_alloc_path();
    if (!path1 || !path2) {
        err = -ENOMEM;
        goto out;
    }
    path1->reada = 1;
    path2->reada = 2;

    node = alloc_backref_node(cache);
    if (!node) {
        err = -ENOMEM;
        goto out;
    }

    node->bytenr = bytenr;
    node->level = level;
    node->lowest = 1;
    cur = node;
again:
    end = 0;
    ptr = 0;
    key.objectid = cur->bytenr;
    key.type = BTRFS_EXTENT_ITEM_KEY;
    key.offset = (u64)-1;

    path1->search_commit_root = 1;
    path1->skip_locking = 1;
    ret = btrfs_search_slot(NULL, rc->extent_root, &key, path1,
                0, 0);
    if (ret < 0) {
        err = ret;
        goto out;
    }
    BUG_ON(!ret || !path1->slots[0]);

    path1->slots[0]--;

    WARN_ON(cur->checked);
    if (!list_empty(&cur->upper)) {
        /*
         * the backref was added previously when processing
         * backref of type BTRFS_TREE_BLOCK_REF_KEY
         */
        BUG_ON(!list_is_singular(&cur->upper));
        edge = list_entry(cur->upper.next, struct backref_edge,
                  list[LOWER]);
        BUG_ON(!list_empty(&edge->list[UPPER]));
        exist = edge->node[UPPER];
        /*
         * add the upper level block to pending list if we need
         * check its backrefs
         */
        if (!exist->checked)
            list_add_tail(&edge->list[UPPER], &list);
    } else {
        exist = NULL;
    }

    while (1) {
        cond_resched();
        eb = path1->nodes[0];

        if (ptr >= end) {
            if (path1->slots[0] >= btrfs_header_nritems(eb)) {
                ret = btrfs_next_leaf(rc->extent_root, path1);
                if (ret < 0) {
                    err = ret;
                    goto out;
                }
                if (ret > 0)
                    break;
                eb = path1->nodes[0];
            }

            btrfs_item_key_to_cpu(eb, &key, path1->slots[0]);
            if (key.objectid != cur->bytenr) {
                WARN_ON(exist);
                break;
            }

            if (key.type == BTRFS_EXTENT_ITEM_KEY) {
                ret = find_inline_backref(eb, path1->slots[0],
                              &ptr, &end);
                if (ret)
                    goto next;
            }
        }

        if (ptr < end) {
            /* update key for inline back ref */
            struct btrfs_extent_inline_ref *iref;
            iref = (struct btrfs_extent_inline_ref *)ptr;
            key.type = btrfs_extent_inline_ref_type(eb, iref);
            key.offset = btrfs_extent_inline_ref_offset(eb, iref);
            WARN_ON(key.type != BTRFS_TREE_BLOCK_REF_KEY &&
                key.type != BTRFS_SHARED_BLOCK_REF_KEY);
        }

        if (exist &&
            ((key.type == BTRFS_TREE_BLOCK_REF_KEY &&
              exist->owner == key.offset) ||
             (key.type == BTRFS_SHARED_BLOCK_REF_KEY &&
              exist->bytenr == key.offset))) {
            exist = NULL;
            goto next;
        }

#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
        if (key.type == BTRFS_SHARED_BLOCK_REF_KEY ||
            key.type == BTRFS_EXTENT_REF_V0_KEY) {
            if (key.type == BTRFS_EXTENT_REF_V0_KEY) {
                struct btrfs_extent_ref_v0 *ref0;
                ref0 = btrfs_item_ptr(eb, path1->slots[0],
                        struct btrfs_extent_ref_v0);
                if (key.objectid == key.offset) {
                    root = find_tree_root(rc, eb, ref0);
                    if (root && !should_ignore_root(root))
                        cur->root = root;
                    else
                        list_add(&cur->list, &useless);
                    break;
                }
                if (is_cowonly_root(btrfs_ref_root_v0(eb,
                                      ref0)))
                    cur->cowonly = 1;
            }
#else
        BUG_ON(key.type == BTRFS_EXTENT_REF_V0_KEY);
        if (key.type == BTRFS_SHARED_BLOCK_REF_KEY) {
#endif
            if (key.objectid == key.offset) {
                /*
                 * only root blocks of reloc trees use
                 * backref of this type.
                 */
                root = find_reloc_root(rc, cur->bytenr);
                BUG_ON(!root);
                cur->root = root;
                break;
            }

            edge = alloc_backref_edge(cache);
            if (!edge) {
                err = -ENOMEM;
                goto out;
            }
            rb_node = tree_search(&cache->rb_root, key.offset);
            if (!rb_node) {
                upper = alloc_backref_node(cache);
                if (!upper) {
                    free_backref_edge(cache, edge);
                    err = -ENOMEM;
                    goto out;
                }
                upper->bytenr = key.offset;
                upper->level = cur->level + 1;
                /*
                 *  backrefs for the upper level block isn't
                 *  cached, add the block to pending list
                 */
                list_add_tail(&edge->list[UPPER], &list);
            } else {
                upper = rb_entry(rb_node, struct backref_node,
                         rb_node);
                BUG_ON(!upper->checked);
                INIT_LIST_HEAD(&edge->list[UPPER]);
            }
            list_add_tail(&edge->list[LOWER], &cur->upper);
            edge->node[LOWER] = cur;
            edge->node[UPPER] = upper;

            goto next;
        } else if (key.type != BTRFS_TREE_BLOCK_REF_KEY) {
            goto next;
        }

        /* key.type == BTRFS_TREE_BLOCK_REF_KEY */
        root = read_fs_root(rc->extent_root->fs_info, key.offset);
        if (IS_ERR(root)) {
            err = PTR_ERR(root);
            goto out;
        }

        if (!root->ref_cows)
            cur->cowonly = 1;

        if (btrfs_root_level(&root->root_item) == cur->level) {
            /* tree root */
            BUG_ON(btrfs_root_bytenr(&root->root_item) !=
                   cur->bytenr);
            if (should_ignore_root(root))
                list_add(&cur->list, &useless);
            else
                cur->root = root;
            break;
        }

        level = cur->level + 1;

        /*
         * searching the tree to find upper level blocks
         * reference the block.
         */
        path2->search_commit_root = 1;
        path2->skip_locking = 1;
        path2->lowest_level = level;
        ret = btrfs_search_slot(NULL, root, node_key, path2, 0, 0);
        path2->lowest_level = 0;
        if (ret < 0) {
            err = ret;
            goto out;
        }
        if (ret > 0 && path2->slots[level] > 0)
            path2->slots[level]--;

        eb = path2->nodes[level];
        WARN_ON(btrfs_node_blockptr(eb, path2->slots[level]) !=
            cur->bytenr);

        lower = cur;
        for (; level < BTRFS_MAX_LEVEL; level++) {
            if (!path2->nodes[level]) {
                BUG_ON(btrfs_root_bytenr(&root->root_item) !=
                       lower->bytenr);
                if (should_ignore_root(root))
                    list_add(&lower->list, &useless);
                else
                    lower->root = root;
                break;
            }

            edge = alloc_backref_edge(cache);
            if (!edge) {
                err = -ENOMEM;
                goto out;
            }

            eb = path2->nodes[level];
            rb_node = tree_search(&cache->rb_root, eb->start);
            if (!rb_node) {
                upper = alloc_backref_node(cache);
                if (!upper) {
                    free_backref_edge(cache, edge);
                    err = -ENOMEM;
                    goto out;
                }
                upper->bytenr = eb->start;
                upper->owner = btrfs_header_owner(eb);
                upper->level = lower->level + 1;
                if (!root->ref_cows)
                    upper->cowonly = 1;

                /*
                 * if we know the block isn't shared
                 * we can void checking its backrefs.
                 */
                if (btrfs_block_can_be_shared(root, eb))
                    upper->checked = 0;
                else
                    upper->checked = 1;

                /*
                 * add the block to pending list if we
                 * need check its backrefs. only block
                 * at 'cur->level + 1' is added to the
                 * tail of pending list. this guarantees
                 * we check backrefs from lower level
                 * blocks to upper level blocks.
                 */
                if (!upper->checked &&
                    level == cur->level + 1) {
                    list_add_tail(&edge->list[UPPER],
                              &list);
                } else
                    INIT_LIST_HEAD(&edge->list[UPPER]);
            } else {
                upper = rb_entry(rb_node, struct backref_node,
                         rb_node);
                BUG_ON(!upper->checked);
                INIT_LIST_HEAD(&edge->list[UPPER]);
                if (!upper->owner)
                    upper->owner = btrfs_header_owner(eb);
            }
            list_add_tail(&edge->list[LOWER], &lower->upper);
            edge->node[LOWER] = lower;
            edge->node[UPPER] = upper;

            if (rb_node)
                break;
            lower = upper;
            upper = NULL;
        }
        btrfs_release_path(path2);
next:
        if (ptr < end) {
            ptr += btrfs_extent_inline_ref_size(key.type);
            if (ptr >= end) {
                WARN_ON(ptr > end);
                ptr = 0;
                end = 0;
            }
        }
        if (ptr >= end)
            path1->slots[0]++;
    }
    btrfs_release_path(path1);

    cur->checked = 1;
    WARN_ON(exist);

    /* the pending list isn't empty, take the first block to process */
    if (!list_empty(&list)) {
        edge = list_entry(list.next, struct backref_edge, list[UPPER]);
        list_del_init(&edge->list[UPPER]);
        cur = edge->node[UPPER];
        goto again;
    }

    /*
     * everything goes well, connect backref nodes and insert backref nodes
     * into the cache.
     */
    BUG_ON(!node->checked);
    cowonly = node->cowonly;
    if (!cowonly) {
        rb_node = tree_insert(&cache->rb_root, node->bytenr,
                      &node->rb_node);
        if (rb_node)
            backref_tree_panic(rb_node, -EEXIST, node->bytenr);
        list_add_tail(&node->lower, &cache->leaves);
    }

    list_for_each_entry(edge, &node->upper, list[LOWER])
        list_add_tail(&edge->list[UPPER], &list);

    while (!list_empty(&list)) {
        edge = list_entry(list.next, struct backref_edge, list[UPPER]);
        list_del_init(&edge->list[UPPER]);
        upper = edge->node[UPPER];
        if (upper->detached) {
            list_del(&edge->list[LOWER]);
            lower = edge->node[LOWER];
            free_backref_edge(cache, edge);
            if (list_empty(&lower->upper))
                list_add(&lower->list, &useless);
            continue;
        }

        if (!RB_EMPTY_NODE(&upper->rb_node)) {
            if (upper->lowest) {
                list_del_init(&upper->lower);
                upper->lowest = 0;
            }

            list_add_tail(&edge->list[UPPER], &upper->lower);
            continue;
        }

        BUG_ON(!upper->checked);
        BUG_ON(cowonly != upper->cowonly);
        if (!cowonly) {
            rb_node = tree_insert(&cache->rb_root, upper->bytenr,
                          &upper->rb_node);
            if (rb_node)
                backref_tree_panic(rb_node, -EEXIST,
                           upper->bytenr);
        }

        list_add_tail(&edge->list[UPPER], &upper->lower);

        list_for_each_entry(edge, &upper->upper, list[LOWER])
            list_add_tail(&edge->list[UPPER], &list);
    }
    /*
     * process useless backref nodes. backref nodes for tree leaves
     * are deleted from the cache. backref nodes for upper level
     * tree blocks are left in the cache to avoid unnecessary backref
     * lookup.
     */
    while (!list_empty(&useless)) {
        upper = list_entry(useless.next, struct backref_node, list);
        list_del_init(&upper->list);
        BUG_ON(!list_empty(&upper->upper));
        if (upper == node)
            node = NULL;
        if (upper->lowest) {
            list_del_init(&upper->lower);
            upper->lowest = 0;
        }
        while (!list_empty(&upper->lower)) {
            edge = list_entry(upper->lower.next,
                      struct backref_edge, list[UPPER]);
            list_del(&edge->list[UPPER]);
            list_del(&edge->list[LOWER]);
            lower = edge->node[LOWER];
            free_backref_edge(cache, edge);

            if (list_empty(&lower->upper))
                list_add(&lower->list, &useless);
        }
        __mark_block_processed(rc, upper);
        if (upper->level > 0) {
            list_add(&upper->list, &cache->detached);
            upper->detached = 1;
        } else {
            rb_erase(&upper->rb_node, &cache->rb_root);
            free_backref_node(cache, upper);
        }
    }
out:
    btrfs_free_path(path1);
    btrfs_free_path(path2);
    if (err) {
        while (!list_empty(&useless)) {
            lower = list_entry(useless.next,
                       struct backref_node, upper);
            list_del_init(&lower->upper);
        }
        upper = node;
        INIT_LIST_HEAD(&list);
        while (upper) {
            if (RB_EMPTY_NODE(&upper->rb_node)) {
                list_splice_tail(&upper->upper, &list);
                free_backref_node(cache, upper);
            }

            if (list_empty(&list))
                break;

            edge = list_entry(list.next, struct backref_edge,
                      list[LOWER]);
            list_del(&edge->list[LOWER]);
            upper = edge->node[UPPER];
            free_backref_edge(cache, edge);
        }
        return ERR_PTR(err);
    }
    BUG_ON(node && node->detached);
    return node;
}

/*
 * helper to add backref node for the newly created snapshot.
 * the backref node is created by cloning backref node that
 * corresponds to root of source tree
 */
static int clone_backref_node(struct btrfs_trans_handle *trans,
                  struct reloc_control *rc,
                  struct btrfs_root *src,
                  struct btrfs_root *dest)
{
    struct btrfs_root *reloc_root = src->reloc_root;
    struct backref_cache *cache = &rc->backref_cache;
    struct backref_node *node = NULL;
    struct backref_node *new_node;
    struct backref_edge *edge;
    struct backref_edge *new_edge;
    struct rb_node *rb_node;

    if (cache->last_trans > 0)
        update_backref_cache(trans, cache);

    rb_node = tree_search(&cache->rb_root, src->commit_root->start);
    if (rb_node) {
        node = rb_entry(rb_node, struct backref_node, rb_node);
        if (node->detached)
            node = NULL;
        else
            BUG_ON(node->new_bytenr != reloc_root->node->start);
    }

    if (!node) {
        rb_node = tree_search(&cache->rb_root,
                      reloc_root->commit_root->start);
        if (rb_node) {
            node = rb_entry(rb_node, struct backref_node,
                    rb_node);
            BUG_ON(node->detached);
        }
    }

    if (!node)
        return 0;

    new_node = alloc_backref_node(cache);
    if (!new_node)
        return -ENOMEM;

    new_node->bytenr = dest->node->start;
    new_node->level = node->level;
    new_node->lowest = node->lowest;
    new_node->checked = 1;
    new_node->root = dest;

    if (!node->lowest) {
        list_for_each_entry(edge, &node->lower, list[UPPER]) {
            new_edge = alloc_backref_edge(cache);
            if (!new_edge)
                goto fail;

            new_edge->node[UPPER] = new_node;
            new_edge->node[LOWER] = edge->node[LOWER];
            list_add_tail(&new_edge->list[UPPER],
                      &new_node->lower);
        }
    } else {
        list_add_tail(&new_node->lower, &cache->leaves);
    }

    rb_node = tree_insert(&cache->rb_root, new_node->bytenr,
                  &new_node->rb_node);
    if (rb_node)
        backref_tree_panic(rb_node, -EEXIST, new_node->bytenr);

    if (!new_node->lowest) {
        list_for_each_entry(new_edge, &new_node->lower, list[UPPER]) {
            list_add_tail(&new_edge->list[LOWER],
                      &new_edge->node[LOWER]->upper);
        }
    }
    return 0;
fail:
    while (!list_empty(&new_node->lower)) {
        new_edge = list_entry(new_node->lower.next,
                      struct backref_edge, list[UPPER]);
        list_del(&new_edge->list[UPPER]);
        free_backref_edge(cache, new_edge);
    }
    free_backref_node(cache, new_node);
    return -ENOMEM;
}

/*
 * helper to add 'address of tree root -> reloc tree' mapping
 */
static int __must_check __add_reloc_root(struct btrfs_root *root)
{
    struct rb_node *rb_node;
    struct mapping_node *node;
    struct reloc_control *rc = root->fs_info->reloc_ctl;

    node = kmalloc(sizeof(*node), GFP_NOFS);
    if (!node)
        return -ENOMEM;

    node->bytenr = root->node->start;
    node->data = root;

    spin_lock(&rc->reloc_root_tree.lock);
    rb_node = tree_insert(&rc->reloc_root_tree.rb_root,
                  node->bytenr, &node->rb_node);
    spin_unlock(&rc->reloc_root_tree.lock);
    if (rb_node) {
        btrfs_panic(root->fs_info, -EEXIST, "Duplicate root found "
                "for start=%llu while inserting into relocation "
                "tree\n", node->bytenr);
        kfree(node);
        return -EEXIST;
    }

    list_add_tail(&root->root_list, &rc->reloc_roots);
    return 0;
}

/*
 * helper to update/delete the 'address of tree root -> reloc tree'
 * mapping
 */
static int __update_reloc_root(struct btrfs_root *root, int del)
{
    struct rb_node *rb_node;
    struct mapping_node *node = NULL;
    struct reloc_control *rc = root->fs_info->reloc_ctl;

    spin_lock(&rc->reloc_root_tree.lock);
    rb_node = tree_search(&rc->reloc_root_tree.rb_root,
                  root->commit_root->start);
    if (rb_node) {
        node = rb_entry(rb_node, struct mapping_node, rb_node);
        rb_erase(&node->rb_node, &rc->reloc_root_tree.rb_root);
    }
    spin_unlock(&rc->reloc_root_tree.lock);

    BUG_ON((struct btrfs_root *)node->data != root);

    if (!del) {
        spin_lock(&rc->reloc_root_tree.lock);
        node->bytenr = root->node->start;
        rb_node = tree_insert(&rc->reloc_root_tree.rb_root,
                      node->bytenr, &node->rb_node);
        spin_unlock(&rc->reloc_root_tree.lock);
        if (rb_node)
            backref_tree_panic(rb_node, -EEXIST, node->bytenr);
    } else {
        spin_lock(&root->fs_info->trans_lock);
        list_del_init(&root->root_list);
        spin_unlock(&root->fs_info->trans_lock);
        kfree(node);
    }
    return 0;
}

static struct btrfs_root *create_reloc_root(struct btrfs_trans_handle *trans,
                    struct btrfs_root *root, u64 objectid)
{
    struct btrfs_root *reloc_root;
    struct extent_buffer *eb;
    struct btrfs_root_item *root_item;
    struct btrfs_key root_key;
    int ret;

    root_item = kmalloc(sizeof(*root_item), GFP_NOFS);
    BUG_ON(!root_item);

    root_key.objectid = BTRFS_TREE_RELOC_OBJECTID;
    root_key.type = BTRFS_ROOT_ITEM_KEY;
    root_key.offset = objectid;

    if (root->root_key.objectid == objectid) {
        /* called by btrfs_init_reloc_root */
        ret = btrfs_copy_root(trans, root, root->commit_root, &eb,
                      BTRFS_TREE_RELOC_OBJECTID);
        BUG_ON(ret);

        btrfs_set_root_last_snapshot(&root->root_item,
                         trans->transid - 1);
    } else {
        /*
         * called by btrfs_reloc_post_snapshot_hook.
         * the source tree is a reloc tree, all tree blocks
         * modified after it was created have RELOC flag
         * set in their headers. so it's OK to not update
         * the 'last_snapshot'.
         */
        ret = btrfs_copy_root(trans, root, root->node, &eb,
                      BTRFS_TREE_RELOC_OBJECTID);
        BUG_ON(ret);
    }

    memcpy(root_item, &root->root_item, sizeof(*root_item));
    btrfs_set_root_bytenr(root_item, eb->start);
    btrfs_set_root_level(root_item, btrfs_header_level(eb));
    btrfs_set_root_generation(root_item, trans->transid);

    if (root->root_key.objectid == objectid) {
        btrfs_set_root_refs(root_item, 0);
        memset(&root_item->drop_progress, 0,
               sizeof(struct btrfs_disk_key));
        root_item->drop_level = 0;
    }

    btrfs_tree_unlock(eb);
    free_extent_buffer(eb);

    ret = btrfs_insert_root(trans, root->fs_info->tree_root,
                &root_key, root_item);
    BUG_ON(ret);
    kfree(root_item);

    reloc_root = btrfs_read_fs_root_no_radix(root->fs_info->tree_root,
                         &root_key);
    BUG_ON(IS_ERR(reloc_root));
    reloc_root->last_trans = trans->transid;
    return reloc_root;
}

/*
 * create reloc tree for a given fs tree. reloc tree is just a
 * snapshot of the fs tree with special root objectid.
 */
int btrfs_init_reloc_root(struct btrfs_trans_handle *trans,
              struct btrfs_root *root)
{
    struct btrfs_root *reloc_root;
    struct reloc_control *rc = root->fs_info->reloc_ctl;
    int clear_rsv = 0;
    int ret;

    if (root->reloc_root) {
        reloc_root = root->reloc_root;
        reloc_root->last_trans = trans->transid;
        return 0;
    }

    if (!rc || !rc->create_reloc_tree ||
        root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID)
        return 0;

    if (!trans->block_rsv) {
        trans->block_rsv = rc->block_rsv;
        clear_rsv = 1;
    }
    reloc_root = create_reloc_root(trans, root, root->root_key.objectid);
    if (clear_rsv)
        trans->block_rsv = NULL;

    ret = __add_reloc_root(reloc_root);
    BUG_ON(ret < 0);
    root->reloc_root = reloc_root;
    return 0;
}

/*
 * update root item of reloc tree
 */
int btrfs_update_reloc_root(struct btrfs_trans_handle *trans,
                struct btrfs_root *root)
{
    struct btrfs_root *reloc_root;
    struct btrfs_root_item *root_item;
    int del = 0;
    int ret;

    if (!root->reloc_root)
        goto out;

    reloc_root = root->reloc_root;
    root_item = &reloc_root->root_item;

    if (root->fs_info->reloc_ctl->merge_reloc_tree &&
        btrfs_root_refs(root_item) == 0) {
        root->reloc_root = NULL;
        del = 1;
    }

    __update_reloc_root(reloc_root, del);

    if (reloc_root->commit_root != reloc_root->node) {
        btrfs_set_root_node(root_item, reloc_root->node);
        free_extent_buffer(reloc_root->commit_root);
        reloc_root->commit_root = btrfs_root_node(reloc_root);
    }

    ret = btrfs_update_root(trans, root->fs_info->tree_root,
                &reloc_root->root_key, root_item);
    BUG_ON(ret);

out:
    return 0;
}

/*
 * helper to find first cached inode with inode number >= objectid
 * in a subvolume
 */
static struct inode *find_next_inode(struct btrfs_root *root, u64 objectid)
{
    struct rb_node *node;
    struct rb_node *prev;
    struct btrfs_inode *entry;
    struct inode *inode;

    spin_lock(&root->inode_lock);
again:
    node = root->inode_tree.rb_node;
    prev = NULL;
    while (node) {
        prev = node;
        entry = rb_entry(node, struct btrfs_inode, rb_node);

        if (objectid < btrfs_ino(&entry->vfs_inode))
            node = node->rb_left;
        else if (objectid > btrfs_ino(&entry->vfs_inode))
            node = node->rb_right;
        else
            break;
    }
    if (!node) {
        while (prev) {
            entry = rb_entry(prev, struct btrfs_inode, rb_node);
            if (objectid <= btrfs_ino(&entry->vfs_inode)) {
                node = prev;
                break;
            }
            prev = rb_next(prev);
        }
    }
    while (node) {
        entry = rb_entry(node, struct btrfs_inode, rb_node);
        inode = igrab(&entry->vfs_inode);
        if (inode) {
            spin_unlock(&root->inode_lock);
            return inode;
        }

        objectid = btrfs_ino(&entry->vfs_inode) + 1;
        if (cond_resched_lock(&root->inode_lock))
            goto again;

        node = rb_next(node);
    }
    spin_unlock(&root->inode_lock);
    return NULL;
}

static int in_block_group(u64 bytenr,
              struct btrfs_block_group_cache *block_group)
{
    if (bytenr >= block_group->key.objectid &&
        bytenr < block_group->key.objectid + block_group->key.offset)
        return 1;
    return 0;
}

/*
 * get new location of data
 */
static int get_new_location(struct inode *reloc_inode, u64 *new_bytenr,
                u64 bytenr, u64 num_bytes)
{
    struct btrfs_root *root = BTRFS_I(reloc_inode)->root;
    struct btrfs_path *path;
    struct btrfs_file_extent_item *fi;
    struct extent_buffer *leaf;
    int ret;

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;

    bytenr -= BTRFS_I(reloc_inode)->index_cnt;
    ret = btrfs_lookup_file_extent(NULL, root, path, btrfs_ino(reloc_inode),
                       bytenr, 0);
    if (ret < 0)
        goto out;
    if (ret > 0) {
        ret = -ENOENT;
        goto out;
    }

    leaf = path->nodes[0];
    fi = btrfs_item_ptr(leaf, path->slots[0],
                struct btrfs_file_extent_item);

    BUG_ON(btrfs_file_extent_offset(leaf, fi) ||
           btrfs_file_extent_compression(leaf, fi) ||
           btrfs_file_extent_encryption(leaf, fi) ||
           btrfs_file_extent_other_encoding(leaf, fi));

    if (num_bytes != btrfs_file_extent_disk_num_bytes(leaf, fi)) {
        ret = 1;
        goto out;
    }

    *new_bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
    ret = 0;
out:
    btrfs_free_path(path);
    return ret;
}

/*
 * update file extent items in the tree leaf to point to
 * the new locations.
 */
static noinline_for_stack
int replace_file_extents(struct btrfs_trans_handle *trans,
             struct reloc_control *rc,
             struct btrfs_root *root,
             struct extent_buffer *leaf)
{
    struct btrfs_key key;
    struct btrfs_file_extent_item *fi;
    struct inode *inode = NULL;
    u64 parent;
    u64 bytenr;
    u64 new_bytenr = 0;
    u64 num_bytes;
    u64 end;
    u32 nritems;
    u32 i;
    int ret;
    int first = 1;
    int dirty = 0;

    if (rc->stage != UPDATE_DATA_PTRS)
        return 0;

    /* reloc trees always use full backref */
    if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID)
        parent = leaf->start;
    else
        parent = 0;

    nritems = btrfs_header_nritems(leaf);
    for (i = 0; i < nritems; i++) {
        cond_resched();
        btrfs_item_key_to_cpu(leaf, &key, i);
        if (key.type != BTRFS_EXTENT_DATA_KEY)
            continue;
        fi = btrfs_item_ptr(leaf, i, struct btrfs_file_extent_item);
        if (btrfs_file_extent_type(leaf, fi) ==
            BTRFS_FILE_EXTENT_INLINE)
            continue;
        bytenr = btrfs_file_extent_disk_bytenr(leaf, fi);
        num_bytes = btrfs_file_extent_disk_num_bytes(leaf, fi);
        if (bytenr == 0)
            continue;
        if (!in_block_group(bytenr, rc->block_group))
            continue;

        /*
         * if we are modifying block in fs tree, wait for readpage
         * to complete and drop the extent cache
         */
        if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID) {
            if (first) {
                inode = find_next_inode(root, key.objectid);
                first = 0;
            } else if (inode && btrfs_ino(inode) < key.objectid) {
                btrfs_add_delayed_iput(inode);
                inode = find_next_inode(root, key.objectid);
            }
            if (inode && btrfs_ino(inode) == key.objectid) {
                end = key.offset +
                      btrfs_file_extent_num_bytes(leaf, fi);
                WARN_ON(!IS_ALIGNED(key.offset,
                            root->sectorsize));
                WARN_ON(!IS_ALIGNED(end, root->sectorsize));
                end--;
                ret = try_lock_extent(&BTRFS_I(inode)->io_tree,
                              key.offset, end);
                if (!ret)
                    continue;

                btrfs_drop_extent_cache(inode, key.offset, end,
                            1);
                unlock_extent(&BTRFS_I(inode)->io_tree,
                          key.offset, end);
            }
        }

        ret = get_new_location(rc->data_inode, &new_bytenr,
                       bytenr, num_bytes);
        if (ret > 0) {
            WARN_ON(1);
            continue;
        }
        BUG_ON(ret < 0);

        btrfs_set_file_extent_disk_bytenr(leaf, fi, new_bytenr);
        dirty = 1;

        key.offset -= btrfs_file_extent_offset(leaf, fi);
        ret = btrfs_inc_extent_ref(trans, root, new_bytenr,
                       num_bytes, parent,
                       btrfs_header_owner(leaf),
                       key.objectid, key.offset, 1);
        BUG_ON(ret);

        ret = btrfs_free_extent(trans, root, bytenr, num_bytes,
                    parent, btrfs_header_owner(leaf),
                    key.objectid, key.offset, 1);
        BUG_ON(ret);
    }
    if (dirty)
        btrfs_mark_buffer_dirty(leaf);
    if (inode)
        btrfs_add_delayed_iput(inode);
    return 0;
}

static noinline_for_stack
int memcmp_node_keys(struct extent_buffer *eb, int slot,
             struct btrfs_path *path, int level)
{
    struct btrfs_disk_key key1;
    struct btrfs_disk_key key2;
    btrfs_node_key(eb, &key1, slot);
    btrfs_node_key(path->nodes[level], &key2, path->slots[level]);
    return memcmp(&key1, &key2, sizeof(key1));
}

/*
 * try to replace tree blocks in fs tree with the new blocks
 * in reloc tree. tree blocks haven't been modified since the
 * reloc tree was create can be replaced.
 *
 * if a block was replaced, level of the block + 1 is returned.
 * if no block got replaced, 0 is returned. if there are other
 * errors, a negative error number is returned.
 */
static noinline_for_stack
int replace_path(struct btrfs_trans_handle *trans,
         struct btrfs_root *dest, struct btrfs_root *src,
         struct btrfs_path *path, struct btrfs_key *next_key,
         int lowest_level, int max_level)
{
    struct extent_buffer *eb;
    struct extent_buffer *parent;
    struct btrfs_key key;
    u64 old_bytenr;
    u64 new_bytenr;
    u64 old_ptr_gen;
    u64 new_ptr_gen;
    u64 last_snapshot;
    u32 blocksize;
    int cow = 0;
    int level;
    int ret;
    int slot;

    BUG_ON(src->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID);
    BUG_ON(dest->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID);

    last_snapshot = btrfs_root_last_snapshot(&src->root_item);
again:
    slot = path->slots[lowest_level];
    btrfs_node_key_to_cpu(path->nodes[lowest_level], &key, slot);

    eb = btrfs_lock_root_node(dest);
    btrfs_set_lock_blocking(eb);
    level = btrfs_header_level(eb);

    if (level < lowest_level) {
        btrfs_tree_unlock(eb);
        free_extent_buffer(eb);
        return 0;
    }

    if (cow) {
        ret = btrfs_cow_block(trans, dest, eb, NULL, 0, &eb);
        BUG_ON(ret);
    }
    btrfs_set_lock_blocking(eb);

    if (next_key) {
        next_key->objectid = (u64)-1;
        next_key->type = (u8)-1;
        next_key->offset = (u64)-1;
    }

    parent = eb;
    while (1) {
        level = btrfs_header_level(parent);
        BUG_ON(level < lowest_level);

        ret = btrfs_bin_search(parent, &key, level, &slot);
        if (ret && slot > 0)
            slot--;

        if (next_key && slot + 1 < btrfs_header_nritems(parent))
            btrfs_node_key_to_cpu(parent, next_key, slot + 1);

        old_bytenr = btrfs_node_blockptr(parent, slot);
        blocksize = btrfs_level_size(dest, level - 1);
        old_ptr_gen = btrfs_node_ptr_generation(parent, slot);

        if (level <= max_level) {
            eb = path->nodes[level];
            new_bytenr = btrfs_node_blockptr(eb,
                            path->slots[level]);
            new_ptr_gen = btrfs_node_ptr_generation(eb,
                            path->slots[level]);
        } else {
            new_bytenr = 0;
            new_ptr_gen = 0;
        }

        if (new_bytenr > 0 && new_bytenr == old_bytenr) {
            WARN_ON(1);
            ret = level;
            break;
        }

        if (new_bytenr == 0 || old_ptr_gen > last_snapshot ||
            memcmp_node_keys(parent, slot, path, level)) {
            if (level <= lowest_level) {
                ret = 0;
                break;
            }

            eb = read_tree_block(dest, old_bytenr, blocksize,
                         old_ptr_gen);
            BUG_ON(!eb);
            btrfs_tree_lock(eb);
            if (cow) {
                ret = btrfs_cow_block(trans, dest, eb, parent,
                              slot, &eb);
                BUG_ON(ret);
            }
            btrfs_set_lock_blocking(eb);

            btrfs_tree_unlock(parent);
            free_extent_buffer(parent);

            parent = eb;
            continue;
        }

        if (!cow) {
            btrfs_tree_unlock(parent);
            free_extent_buffer(parent);
            cow = 1;
            goto again;
        }

        btrfs_node_key_to_cpu(path->nodes[level], &key,
                      path->slots[level]);
        btrfs_release_path(path);

        path->lowest_level = level;
        ret = btrfs_search_slot(trans, src, &key, path, 0, 1);
        path->lowest_level = 0;
        BUG_ON(ret);

        /*
         * swap blocks in fs tree and reloc tree.
         */
        btrfs_set_node_blockptr(parent, slot, new_bytenr);
        btrfs_set_node_ptr_generation(parent, slot, new_ptr_gen);
        btrfs_mark_buffer_dirty(parent);

        btrfs_set_node_blockptr(path->nodes[level],
                    path->slots[level], old_bytenr);
        btrfs_set_node_ptr_generation(path->nodes[level],
                          path->slots[level], old_ptr_gen);
        btrfs_mark_buffer_dirty(path->nodes[level]);

        ret = btrfs_inc_extent_ref(trans, src, old_bytenr, blocksize,
                    path->nodes[level]->start,
                    src->root_key.objectid, level - 1, 0,
                    1);
        BUG_ON(ret);
        ret = btrfs_inc_extent_ref(trans, dest, new_bytenr, blocksize,
                    0, dest->root_key.objectid, level - 1,
                    0, 1);
        BUG_ON(ret);

        ret = btrfs_free_extent(trans, src, new_bytenr, blocksize,
                    path->nodes[level]->start,
                    src->root_key.objectid, level - 1, 0,
                    1);
        BUG_ON(ret);

        ret = btrfs_free_extent(trans, dest, old_bytenr, blocksize,
                    0, dest->root_key.objectid, level - 1,
                    0, 1);
        BUG_ON(ret);

        btrfs_unlock_up_safe(path, 0);

        ret = level;
        break;
    }
    btrfs_tree_unlock(parent);
    free_extent_buffer(parent);
    return ret;
}

/*
 * helper to find next relocated block in reloc tree
 */
static noinline_for_stack
int walk_up_reloc_tree(struct btrfs_root *root, struct btrfs_path *path,
               int *level)
{
    struct extent_buffer *eb;
    int i;
    u64 last_snapshot;
    u32 nritems;

    last_snapshot = btrfs_root_last_snapshot(&root->root_item);

    for (i = 0; i < *level; i++) {
        free_extent_buffer(path->nodes[i]);
        path->nodes[i] = NULL;
    }

    for (i = *level; i < BTRFS_MAX_LEVEL && path->nodes[i]; i++) {
        eb = path->nodes[i];
        nritems = btrfs_header_nritems(eb);
        while (path->slots[i] + 1 < nritems) {
            path->slots[i]++;
            if (btrfs_node_ptr_generation(eb, path->slots[i]) <=
                last_snapshot)
                continue;

            *level = i;
            return 0;
        }
        free_extent_buffer(path->nodes[i]);
        path->nodes[i] = NULL;
    }
    return 1;
}

/*
 * walk down reloc tree to find relocated block of lowest level
 */
static noinline_for_stack
int walk_down_reloc_tree(struct btrfs_root *root, struct btrfs_path *path,
             int *level)
{
    struct extent_buffer *eb = NULL;
    int i;
    u64 bytenr;
    u64 ptr_gen = 0;
    u64 last_snapshot;
    u32 blocksize;
    u32 nritems;

    last_snapshot = btrfs_root_last_snapshot(&root->root_item);

    for (i = *level; i > 0; i--) {
        eb = path->nodes[i];
        nritems = btrfs_header_nritems(eb);
        while (path->slots[i] < nritems) {
            ptr_gen = btrfs_node_ptr_generation(eb, path->slots[i]);
            if (ptr_gen > last_snapshot)
                break;
            path->slots[i]++;
        }
        if (path->slots[i] >= nritems) {
            if (i == *level)
                break;
            *level = i + 1;
            return 0;
        }
        if (i == 1) {
            *level = i;
            return 0;
        }

        bytenr = btrfs_node_blockptr(eb, path->slots[i]);
        blocksize = btrfs_level_size(root, i - 1);
        eb = read_tree_block(root, bytenr, blocksize, ptr_gen);
        BUG_ON(btrfs_header_level(eb) != i - 1);
        path->nodes[i - 1] = eb;
        path->slots[i - 1] = 0;
    }
    return 1;
}

/*
 * invalidate extent cache for file extents whose key in range of
 * [min_key, max_key)
 */
static int invalidate_extent_cache(struct btrfs_root *root,
                   struct btrfs_key *min_key,
                   struct btrfs_key *max_key)
{
    struct inode *inode = NULL;
    u64 objectid;
    u64 start, end;
    u64 ino;

    objectid = min_key->objectid;
    while (1) {
        cond_resched();
        iput(inode);

        if (objectid > max_key->objectid)
            break;

        inode = find_next_inode(root, objectid);
        if (!inode)
            break;
        ino = btrfs_ino(inode);

        if (ino > max_key->objectid) {
            iput(inode);
            break;
        }

        objectid = ino + 1;
        if (!S_ISREG(inode->i_mode))
            continue;

        if (unlikely(min_key->objectid == ino)) {
            if (min_key->type > BTRFS_EXTENT_DATA_KEY)
                continue;
            if (min_key->type < BTRFS_EXTENT_DATA_KEY)
                start = 0;
            else {
                start = min_key->offset;
                WARN_ON(!IS_ALIGNED(start, root->sectorsize));
            }
        } else {
            start = 0;
        }

        if (unlikely(max_key->objectid == ino)) {
            if (max_key->type < BTRFS_EXTENT_DATA_KEY)
                continue;
            if (max_key->type > BTRFS_EXTENT_DATA_KEY) {
                end = (u64)-1;
            } else {
                if (max_key->offset == 0)
                    continue;
                end = max_key->offset;
                WARN_ON(!IS_ALIGNED(end, root->sectorsize));
                end--;
            }
        } else {
            end = (u64)-1;
        }

        /* the lock_extent waits for readpage to complete */
        lock_extent(&BTRFS_I(inode)->io_tree, start, end);
        btrfs_drop_extent_cache(inode, start, end, 1);
        unlock_extent(&BTRFS_I(inode)->io_tree, start, end);
    }
    return 0;
}

static int find_next_key(struct btrfs_path *path, int level,
             struct btrfs_key *key)

{
    while (level < BTRFS_MAX_LEVEL) {
        if (!path->nodes[level])
            break;
        if (path->slots[level] + 1 <
            btrfs_header_nritems(path->nodes[level])) {
            btrfs_node_key_to_cpu(path->nodes[level], key,
                          path->slots[level] + 1);
            return 0;
        }
        level++;
    }
    return 1;
}

/*
 * merge the relocated tree blocks in reloc tree with corresponding
 * fs tree.
 */
static noinline_for_stack int merge_reloc_root(struct reloc_control *rc,
                           struct btrfs_root *root)
{
    LIST_HEAD(inode_list);
    struct btrfs_key key;
    struct btrfs_key next_key;
    struct btrfs_trans_handle *trans;
    struct btrfs_root *reloc_root;
    struct btrfs_root_item *root_item;
    struct btrfs_path *path;
    struct extent_buffer *leaf;
    unsigned long nr;
    int level;
    int max_level;
    int replaced = 0;
    int ret;
    int err = 0;
    u32 min_reserved;

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;
    path->reada = 1;

    reloc_root = root->reloc_root;
    root_item = &reloc_root->root_item;

    if (btrfs_disk_key_objectid(&root_item->drop_progress) == 0) {
        level = btrfs_root_level(root_item);
        extent_buffer_get(reloc_root->node);
        path->nodes[level] = reloc_root->node;
        path->slots[level] = 0;
    } else {
        btrfs_disk_key_to_cpu(&key, &root_item->drop_progress);

        level = root_item->drop_level;
        BUG_ON(level == 0);
        path->lowest_level = level;
        ret = btrfs_search_slot(NULL, reloc_root, &key, path, 0, 0);
        path->lowest_level = 0;
        if (ret < 0) {
            btrfs_free_path(path);
            return ret;
        }

        btrfs_node_key_to_cpu(path->nodes[level], &next_key,
                      path->slots[level]);
        WARN_ON(memcmp(&key, &next_key, sizeof(key)));

        btrfs_unlock_up_safe(path, 0);
    }

    min_reserved = root->nodesize * (BTRFS_MAX_LEVEL - 1) * 2;
    memset(&next_key, 0, sizeof(next_key));

    while (1) {
        trans = btrfs_start_transaction(root, 0);
        BUG_ON(IS_ERR(trans));
        trans->block_rsv = rc->block_rsv;

        ret = btrfs_block_rsv_refill(root, rc->block_rsv, min_reserved);
        if (ret) {
            BUG_ON(ret != -EAGAIN);
            ret = btrfs_commit_transaction(trans, root);
            BUG_ON(ret);
            continue;
        }

        replaced = 0;
        max_level = level;

        ret = walk_down_reloc_tree(reloc_root, path, &level);
        if (ret < 0) {
            err = ret;
            goto out;
        }
        if (ret > 0)
            break;

        if (!find_next_key(path, level, &key) &&
            btrfs_comp_cpu_keys(&next_key, &key) >= 0) {
            ret = 0;
        } else {
            ret = replace_path(trans, root, reloc_root, path,
                       &next_key, level, max_level);
        }
        if (ret < 0) {
            err = ret;
            goto out;
        }

        if (ret > 0) {
            level = ret;
            btrfs_node_key_to_cpu(path->nodes[level], &key,
                          path->slots[level]);
            replaced = 1;
        }

        ret = walk_up_reloc_tree(reloc_root, path, &level);
        if (ret > 0)
            break;

        BUG_ON(level == 0);
        /*
         * save the merging progress in the drop_progress.
         * this is OK since root refs == 1 in this case.
         */
        btrfs_node_key(path->nodes[level], &root_item->drop_progress,
                   path->slots[level]);
        root_item->drop_level = level;

        nr = trans->blocks_used;
        btrfs_end_transaction_throttle(trans, root);

        btrfs_btree_balance_dirty(root, nr);

        if (replaced && rc->stage == UPDATE_DATA_PTRS)
            invalidate_extent_cache(root, &key, &next_key);
    }

    /*
     * handle the case only one block in the fs tree need to be
     * relocated and the block is tree root.
     */
    leaf = btrfs_lock_root_node(root);
    ret = btrfs_cow_block(trans, root, leaf, NULL, 0, &leaf);
    btrfs_tree_unlock(leaf);
    free_extent_buffer(leaf);
    if (ret < 0)
        err = ret;
out:
    btrfs_free_path(path);

    if (err == 0) {
        memset(&root_item->drop_progress, 0,
               sizeof(root_item->drop_progress));
        root_item->drop_level = 0;
        btrfs_set_root_refs(root_item, 0);
        btrfs_update_reloc_root(trans, root);
    }

    nr = trans->blocks_used;
    btrfs_end_transaction_throttle(trans, root);

    btrfs_btree_balance_dirty(root, nr);

    if (replaced && rc->stage == UPDATE_DATA_PTRS)
        invalidate_extent_cache(root, &key, &next_key);

    return err;
}

static noinline_for_stack
int prepare_to_merge(struct reloc_control *rc, int err)
{
    struct btrfs_root *root = rc->extent_root;
    struct btrfs_root *reloc_root;
    struct btrfs_trans_handle *trans;
    LIST_HEAD(reloc_roots);
    u64 num_bytes = 0;
    int ret;

    mutex_lock(&root->fs_info->reloc_mutex);
    rc->merging_rsv_size += root->nodesize * (BTRFS_MAX_LEVEL - 1) * 2;
    rc->merging_rsv_size += rc->nodes_relocated * 2;
    mutex_unlock(&root->fs_info->reloc_mutex);

again:
    if (!err) {
        num_bytes = rc->merging_rsv_size;
        ret = btrfs_block_rsv_add(root, rc->block_rsv, num_bytes);
        if (ret)
            err = ret;
    }

    trans = btrfs_join_transaction(rc->extent_root);
    if (IS_ERR(trans)) {
        if (!err)
            btrfs_block_rsv_release(rc->extent_root,
                        rc->block_rsv, num_bytes);
        return PTR_ERR(trans);
    }

    if (!err) {
        if (num_bytes != rc->merging_rsv_size) {
            btrfs_end_transaction(trans, rc->extent_root);
            btrfs_block_rsv_release(rc->extent_root,
                        rc->block_rsv, num_bytes);
            goto again;
        }
    }

    rc->merge_reloc_tree = 1;

    while (!list_empty(&rc->reloc_roots)) {
        reloc_root = list_entry(rc->reloc_roots.next,
                    struct btrfs_root, root_list);
        list_del_init(&reloc_root->root_list);

        root = read_fs_root(reloc_root->fs_info,
                    reloc_root->root_key.offset);
        BUG_ON(IS_ERR(root));
        BUG_ON(root->reloc_root != reloc_root);

        /*
         * set reference count to 1, so btrfs_recover_relocation
         * knows it should resumes merging
         */
        if (!err)
            btrfs_set_root_refs(&reloc_root->root_item, 1);
        btrfs_update_reloc_root(trans, root);

        list_add(&reloc_root->root_list, &reloc_roots);
    }

    list_splice(&reloc_roots, &rc->reloc_roots);

    if (!err)
        btrfs_commit_transaction(trans, rc->extent_root);
    else
        btrfs_end_transaction(trans, rc->extent_root);
    return err;
}

static noinline_for_stack
int merge_reloc_roots(struct reloc_control *rc)
{
    struct btrfs_root *root;
    struct btrfs_root *reloc_root;
    LIST_HEAD(reloc_roots);
    int found = 0;
    int ret;
again:
    root = rc->extent_root;

    /*
     * this serializes us with btrfs_record_root_in_transaction,
     * we have to make sure nobody is in the middle of
     * adding their roots to the list while we are
     * doing this splice
     */
    mutex_lock(&root->fs_info->reloc_mutex);
    list_splice_init(&rc->reloc_roots, &reloc_roots);
    mutex_unlock(&root->fs_info->reloc_mutex);

    while (!list_empty(&reloc_roots)) {
        found = 1;
        reloc_root = list_entry(reloc_roots.next,
                    struct btrfs_root, root_list);

        if (btrfs_root_refs(&reloc_root->root_item) > 0) {
            root = read_fs_root(reloc_root->fs_info,
                        reloc_root->root_key.offset);
            BUG_ON(IS_ERR(root));
            BUG_ON(root->reloc_root != reloc_root);

            ret = merge_reloc_root(rc, root);
            BUG_ON(ret);
        } else {
            list_del_init(&reloc_root->root_list);
        }
        ret = btrfs_drop_snapshot(reloc_root, rc->block_rsv, 0, 1);
        BUG_ON(ret < 0);
    }

    if (found) {
        found = 0;
        goto again;
    }
    BUG_ON(!RB_EMPTY_ROOT(&rc->reloc_root_tree.rb_root));
    return 0;
}

static void free_block_list(struct rb_root *blocks)
{
    struct tree_block *block;
    struct rb_node *rb_node;
    while ((rb_node = rb_first(blocks))) {
        block = rb_entry(rb_node, struct tree_block, rb_node);
        rb_erase(rb_node, blocks);
        kfree(block);
    }
}

static int record_reloc_root_in_trans(struct btrfs_trans_handle *trans,
                      struct btrfs_root *reloc_root)
{
    struct btrfs_root *root;

    if (reloc_root->last_trans == trans->transid)
        return 0;

    root = read_fs_root(reloc_root->fs_info, reloc_root->root_key.offset);
    BUG_ON(IS_ERR(root));
    BUG_ON(root->reloc_root != reloc_root);

    return btrfs_record_root_in_trans(trans, root);
}

static noinline_for_stack
struct btrfs_root *select_reloc_root(struct btrfs_trans_handle *trans,
                     struct reloc_control *rc,
                     struct backref_node *node,
                     struct backref_edge *edges[], int *nr)
{
    struct backref_node *next;
    struct btrfs_root *root;
    int index = 0;

    next = node;
    while (1) {
        cond_resched();
        next = walk_up_backref(next, edges, &index);
        root = next->root;
        BUG_ON(!root);
        BUG_ON(!root->ref_cows);

        if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID) {
            record_reloc_root_in_trans(trans, root);
            break;
        }

        btrfs_record_root_in_trans(trans, root);
        root = root->reloc_root;

        if (next->new_bytenr != root->node->start) {
            BUG_ON(next->new_bytenr);
            BUG_ON(!list_empty(&next->list));
            next->new_bytenr = root->node->start;
            next->root = root;
            list_add_tail(&next->list,
                      &rc->backref_cache.changed);
            __mark_block_processed(rc, next);
            break;
        }

        WARN_ON(1);
        root = NULL;
        next = walk_down_backref(edges, &index);
        if (!next || next->level <= node->level)
            break;
    }
    if (!root)
        return NULL;

    *nr = index;
    next = node;
    /* setup backref node path for btrfs_reloc_cow_block */
    while (1) {
        rc->backref_cache.path[next->level] = next;
        if (--index < 0)
            break;
        next = edges[index]->node[UPPER];
    }
    return root;
}

/*
 * select a tree root for relocation. return NULL if the block
 * is reference counted. we should use do_relocation() in this
 * case. return a tree root pointer if the block isn't reference
 * counted. return -ENOENT if the block is root of reloc tree.
 */
static noinline_for_stack
struct btrfs_root *select_one_root(struct btrfs_trans_handle *trans,
                   struct backref_node *node)
{
    struct backref_node *next;
    struct btrfs_root *root;
    struct btrfs_root *fs_root = NULL;
    struct backref_edge *edges[BTRFS_MAX_LEVEL - 1];
    int index = 0;

    next = node;
    while (1) {
        cond_resched();
        next = walk_up_backref(next, edges, &index);
        root = next->root;
        BUG_ON(!root);

        /* no other choice for non-references counted tree */
        if (!root->ref_cows)
            return root;

        if (root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID)
            fs_root = root;

        if (next != node)
            return NULL;

        next = walk_down_backref(edges, &index);
        if (!next || next->level <= node->level)
            break;
    }

    if (!fs_root)
        return ERR_PTR(-ENOENT);
    return fs_root;
}

static noinline_for_stack
u64 calcu_metadata_size(struct reloc_control *rc,
            struct backref_node *node, int reserve)
{
    struct backref_node *next = node;
    struct backref_edge *edge;
    struct backref_edge *edges[BTRFS_MAX_LEVEL - 1];
    u64 num_bytes = 0;
    int index = 0;

    BUG_ON(reserve && node->processed);

    while (next) {
        cond_resched();
        while (1) {
            if (next->processed && (reserve || next != node))
                break;

            num_bytes += btrfs_level_size(rc->extent_root,
                              next->level);

            if (list_empty(&next->upper))
                break;

            edge = list_entry(next->upper.next,
                      struct backref_edge, list[LOWER]);
            edges[index++] = edge;
            next = edge->node[UPPER];
        }
        next = walk_down_backref(edges, &index);
    }
    return num_bytes;
}

static int reserve_metadata_space(struct btrfs_trans_handle *trans,
                  struct reloc_control *rc,
                  struct backref_node *node)
{
    struct btrfs_root *root = rc->extent_root;
    u64 num_bytes;
    int ret;

    num_bytes = calcu_metadata_size(rc, node, 1) * 2;

    trans->block_rsv = rc->block_rsv;
    ret = btrfs_block_rsv_add(root, rc->block_rsv, num_bytes);
    if (ret) {
        if (ret == -EAGAIN)
            rc->commit_transaction = 1;
        return ret;
    }

    return 0;
}

static void release_metadata_space(struct reloc_control *rc,
                   struct backref_node *node)
{
    u64 num_bytes = calcu_metadata_size(rc, node, 0) * 2;
    btrfs_block_rsv_release(rc->extent_root, rc->block_rsv, num_bytes);
}

/*
 * relocate a block tree, and then update pointers in upper level
 * blocks that reference the block to point to the new location.
 *
 * if called by link_to_upper, the block has already been relocated.
 * in that case this function just updates pointers.
 */
static int do_relocation(struct btrfs_trans_handle *trans,
             struct reloc_control *rc,
             struct backref_node *node,
             struct btrfs_key *key,
             struct btrfs_path *path, int lowest)
{
    struct backref_node *upper;
    struct backref_edge *edge;
    struct backref_edge *edges[BTRFS_MAX_LEVEL - 1];
    struct btrfs_root *root;
    struct extent_buffer *eb;
    u32 blocksize;
    u64 bytenr;
    u64 generation;
    int nr;
    int slot;
    int ret;
    int err = 0;

    BUG_ON(lowest && node->eb);

    path->lowest_level = node->level + 1;
    rc->backref_cache.path[node->level] = node;
    list_for_each_entry(edge, &node->upper, list[LOWER]) {
        cond_resched();

        upper = edge->node[UPPER];
        root = select_reloc_root(trans, rc, upper, edges, &nr);
        BUG_ON(!root);

        if (upper->eb && !upper->locked) {
            if (!lowest) {
                ret = btrfs_bin_search(upper->eb, key,
                               upper->level, &slot);
                BUG_ON(ret);
                bytenr = btrfs_node_blockptr(upper->eb, slot);
                if (node->eb->start == bytenr)
                    goto next;
            }
            drop_node_buffer(upper);
        }

        if (!upper->eb) {
            ret = btrfs_search_slot(trans, root, key, path, 0, 1);
            if (ret < 0) {
                err = ret;
                break;
            }
            BUG_ON(ret > 0);

            if (!upper->eb) {
                upper->eb = path->nodes[upper->level];
                path->nodes[upper->level] = NULL;
            } else {
                BUG_ON(upper->eb != path->nodes[upper->level]);
            }

            upper->locked = 1;
            path->locks[upper->level] = 0;

            slot = path->slots[upper->level];
            btrfs_release_path(path);
        } else {
            ret = btrfs_bin_search(upper->eb, key, upper->level,
                           &slot);
            BUG_ON(ret);
        }

        bytenr = btrfs_node_blockptr(upper->eb, slot);
        if (lowest) {
            BUG_ON(bytenr != node->bytenr);
        } else {
            if (node->eb->start == bytenr)
                goto next;
        }

        blocksize = btrfs_level_size(root, node->level);
        generation = btrfs_node_ptr_generation(upper->eb, slot);
        eb = read_tree_block(root, bytenr, blocksize, generation);
        if (!eb) {
            err = -EIO;
            goto next;
        }
        btrfs_tree_lock(eb);
        btrfs_set_lock_blocking(eb);

        if (!node->eb) {
            ret = btrfs_cow_block(trans, root, eb, upper->eb,
                          slot, &eb);
            btrfs_tree_unlock(eb);
            free_extent_buffer(eb);
            if (ret < 0) {
                err = ret;
                goto next;
            }
            BUG_ON(node->eb != eb);
        } else {
            btrfs_set_node_blockptr(upper->eb, slot,
                        node->eb->start);
            btrfs_set_node_ptr_generation(upper->eb, slot,
                              trans->transid);
            btrfs_mark_buffer_dirty(upper->eb);

            ret = btrfs_inc_extent_ref(trans, root,
                        node->eb->start, blocksize,
                        upper->eb->start,
                        btrfs_header_owner(upper->eb),
                        node->level, 0, 1);
            BUG_ON(ret);

            ret = btrfs_drop_subtree(trans, root, eb, upper->eb);
            BUG_ON(ret);
        }
next:
        if (!upper->pending)
            drop_node_buffer(upper);
        else
            unlock_node_buffer(upper);
        if (err)
            break;
    }

    if (!err && node->pending) {
        drop_node_buffer(node);
        list_move_tail(&node->list, &rc->backref_cache.changed);
        node->pending = 0;
    }

    path->lowest_level = 0;
    BUG_ON(err == -ENOSPC);
    return err;
}

static int link_to_upper(struct btrfs_trans_handle *trans,
             struct reloc_control *rc,
             struct backref_node *node,
             struct btrfs_path *path)
{
    struct btrfs_key key;

    btrfs_node_key_to_cpu(node->eb, &key, 0);
    return do_relocation(trans, rc, node, &key, path, 0);
}

static int finish_pending_nodes(struct btrfs_trans_handle *trans,
                struct reloc_control *rc,
                struct btrfs_path *path, int err)
{
    LIST_HEAD(list);
    struct backref_cache *cache = &rc->backref_cache;
    struct backref_node *node;
    int level;
    int ret;

    for (level = 0; level < BTRFS_MAX_LEVEL; level++) {
        while (!list_empty(&cache->pending[level])) {
            node = list_entry(cache->pending[level].next,
                      struct backref_node, list);
            list_move_tail(&node->list, &list);
            BUG_ON(!node->pending);

            if (!err) {
                ret = link_to_upper(trans, rc, node, path);
                if (ret < 0)
                    err = ret;
            }
        }
        list_splice_init(&list, &cache->pending[level]);
    }
    return err;
}

static void mark_block_processed(struct reloc_control *rc,
                 u64 bytenr, u32 blocksize)
{
    set_extent_bits(&rc->processed_blocks, bytenr, bytenr + blocksize - 1,
            EXTENT_DIRTY, GFP_NOFS);
}

static void __mark_block_processed(struct reloc_control *rc,
                   struct backref_node *node)
{
    u32 blocksize;
    if (node->level == 0 ||
        in_block_group(node->bytenr, rc->block_group)) {
        blocksize = btrfs_level_size(rc->extent_root, node->level);
        mark_block_processed(rc, node->bytenr, blocksize);
    }
    node->processed = 1;
}

/*
 * mark a block and all blocks directly/indirectly reference the block
 * as processed.
 */
static void update_processed_blocks(struct reloc_control *rc,
                    struct backref_node *node)
{
    struct backref_node *next = node;
    struct backref_edge *edge;
    struct backref_edge *edges[BTRFS_MAX_LEVEL - 1];
    int index = 0;

    while (next) {
        cond_resched();
        while (1) {
            if (next->processed)
                break;

            __mark_block_processed(rc, next);

            if (list_empty(&next->upper))
                break;

            edge = list_entry(next->upper.next,
                      struct backref_edge, list[LOWER]);
            edges[index++] = edge;
            next = edge->node[UPPER];
        }
        next = walk_down_backref(edges, &index);
    }
}

static int tree_block_processed(u64 bytenr, u32 blocksize,
                struct reloc_control *rc)
{
    if (test_range_bit(&rc->processed_blocks, bytenr,
               bytenr + blocksize - 1, EXTENT_DIRTY, 1, NULL))
        return 1;
    return 0;
}

static int get_tree_block_key(struct reloc_control *rc,
                  struct tree_block *block)
{
    struct extent_buffer *eb;

    BUG_ON(block->key_ready);
    eb = read_tree_block(rc->extent_root, block->bytenr,
                 block->key.objectid, block->key.offset);
    BUG_ON(!eb);
    WARN_ON(btrfs_header_level(eb) != block->level);
    if (block->level == 0)
        btrfs_item_key_to_cpu(eb, &block->key, 0);
    else
        btrfs_node_key_to_cpu(eb, &block->key, 0);
    free_extent_buffer(eb);
    block->key_ready = 1;
    return 0;
}

static int reada_tree_block(struct reloc_control *rc,
                struct tree_block *block)
{
    BUG_ON(block->key_ready);
    readahead_tree_block(rc->extent_root, block->bytenr,
                 block->key.objectid, block->key.offset);
    return 0;
}

/*
 * helper function to relocate a tree block
 */
static int relocate_tree_block(struct btrfs_trans_handle *trans,
                struct reloc_control *rc,
                struct backref_node *node,
                struct btrfs_key *key,
                struct btrfs_path *path)
{
    struct btrfs_root *root;
    int release = 0;
    int ret = 0;

    if (!node)
        return 0;

    BUG_ON(node->processed);
    root = select_one_root(trans, node);
    if (root == ERR_PTR(-ENOENT)) {
        update_processed_blocks(rc, node);
        goto out;
    }

    if (!root || root->ref_cows) {
        ret = reserve_metadata_space(trans, rc, node);
        if (ret)
            goto out;
        release = 1;
    }

    if (root) {
        if (root->ref_cows) {
            BUG_ON(node->new_bytenr);
            BUG_ON(!list_empty(&node->list));
            btrfs_record_root_in_trans(trans, root);
            root = root->reloc_root;
            node->new_bytenr = root->node->start;
            node->root = root;
            list_add_tail(&node->list, &rc->backref_cache.changed);
        } else {
            path->lowest_level = node->level;
            ret = btrfs_search_slot(trans, root, key, path, 0, 1);
            btrfs_release_path(path);
            if (ret > 0)
                ret = 0;
        }
        if (!ret)
            update_processed_blocks(rc, node);
    } else {
        ret = do_relocation(trans, rc, node, key, path, 1);
    }
out:
    if (ret || node->level == 0 || node->cowonly) {
        if (release)
            release_metadata_space(rc, node);
        remove_backref_node(&rc->backref_cache, node);
    }
    return ret;
}

/*
 * relocate a list of blocks
 */
static noinline_for_stack
int relocate_tree_blocks(struct btrfs_trans_handle *trans,
             struct reloc_control *rc, struct rb_root *blocks)
{
    struct backref_node *node;
    struct btrfs_path *path;
    struct tree_block *block;
    struct rb_node *rb_node;
    int ret;
    int err = 0;

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;

    rb_node = rb_first(blocks);
    while (rb_node) {
        block = rb_entry(rb_node, struct tree_block, rb_node);
        if (!block->key_ready)
            reada_tree_block(rc, block);
        rb_node = rb_next(rb_node);
    }

    rb_node = rb_first(blocks);
    while (rb_node) {
        block = rb_entry(rb_node, struct tree_block, rb_node);
        if (!block->key_ready)
            get_tree_block_key(rc, block);
        rb_node = rb_next(rb_node);
    }

    rb_node = rb_first(blocks);
    while (rb_node) {
        block = rb_entry(rb_node, struct tree_block, rb_node);

        node = build_backref_tree(rc, &block->key,
                      block->level, block->bytenr);
        if (IS_ERR(node)) {
            err = PTR_ERR(node);
            goto out;
        }

        ret = relocate_tree_block(trans, rc, node, &block->key,
                      path);
        if (ret < 0) {
            if (ret != -EAGAIN || rb_node == rb_first(blocks))
                err = ret;
            goto out;
        }
        rb_node = rb_next(rb_node);
    }
out:
    free_block_list(blocks);
    err = finish_pending_nodes(trans, rc, path, err);

    btrfs_free_path(path);
    return err;
}

static noinline_for_stack
int prealloc_file_extent_cluster(struct inode *inode,
                 struct file_extent_cluster *cluster)
{
    u64 alloc_hint = 0;
    u64 start;
    u64 end;
    u64 offset = BTRFS_I(inode)->index_cnt;
    u64 num_bytes;
    int nr = 0;
    int ret = 0;

    BUG_ON(cluster->start != cluster->boundary[0]);
    mutex_lock(&inode->i_mutex);

    ret = btrfs_check_data_free_space(inode, cluster->end +
                      1 - cluster->start);
    if (ret)
        goto out;

    while (nr < cluster->nr) {
        start = cluster->boundary[nr] - offset;
        if (nr + 1 < cluster->nr)
            end = cluster->boundary[nr + 1] - 1 - offset;
        else
            end = cluster->end - offset;

        lock_extent(&BTRFS_I(inode)->io_tree, start, end);
        num_bytes = end + 1 - start;
        ret = btrfs_prealloc_file_range(inode, 0, start,
                        num_bytes, num_bytes,
                        end + 1, &alloc_hint);
        unlock_extent(&BTRFS_I(inode)->io_tree, start, end);
        if (ret)
            break;
        nr++;
    }
    btrfs_free_reserved_data_space(inode, cluster->end +
                       1 - cluster->start);
out:
    mutex_unlock(&inode->i_mutex);
    return ret;
}

static noinline_for_stack
int setup_extent_mapping(struct inode *inode, u64 start, u64 end,
             u64 block_start)
{
    struct btrfs_root *root = BTRFS_I(inode)->root;
    struct extent_map_tree *em_tree = &BTRFS_I(inode)->extent_tree;
    struct extent_map *em;
    int ret = 0;

    em = alloc_extent_map();
    if (!em)
        return -ENOMEM;

    em->start = start;
    em->len = end + 1 - start;
    em->block_len = em->len;
    em->block_start = block_start;
    em->bdev = root->fs_info->fs_devices->latest_bdev;
    set_bit(EXTENT_FLAG_PINNED, &em->flags);

    lock_extent(&BTRFS_I(inode)->io_tree, start, end);
    while (1) {
        write_lock(&em_tree->lock);
        ret = add_extent_mapping(em_tree, em);
        write_unlock(&em_tree->lock);
        if (ret != -EEXIST) {
            free_extent_map(em);
            break;
        }
        btrfs_drop_extent_cache(inode, start, end, 0);
    }
    unlock_extent(&BTRFS_I(inode)->io_tree, start, end);
    return ret;
}

static int relocate_file_extent_cluster(struct inode *inode,
                    struct file_extent_cluster *cluster)
{
    u64 page_start;
    u64 page_end;
    u64 offset = BTRFS_I(inode)->index_cnt;
    unsigned long index;
    unsigned long last_index;
    struct page *page;
    struct file_ra_state *ra;
    gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
    int nr = 0;
    int ret = 0;

    if (!cluster->nr)
        return 0;

    ra = kzalloc(sizeof(*ra), GFP_NOFS);
    if (!ra)
        return -ENOMEM;

    ret = prealloc_file_extent_cluster(inode, cluster);
    if (ret)
        goto out;

    file_ra_state_init(ra, inode->i_mapping);

    ret = setup_extent_mapping(inode, cluster->start - offset,
                   cluster->end - offset, cluster->start);
    if (ret)
        goto out;

    index = (cluster->start - offset) >> PAGE_CACHE_SHIFT;
    last_index = (cluster->end - offset) >> PAGE_CACHE_SHIFT;
    while (index <= last_index) {
        ret = btrfs_delalloc_reserve_metadata(inode, PAGE_CACHE_SIZE);
        if (ret)
            goto out;

        page = find_lock_page(inode->i_mapping, index);
        if (!page) {
            page_cache_sync_readahead(inode->i_mapping,
                          ra, NULL, index,
                          last_index + 1 - index);
            page = find_or_create_page(inode->i_mapping, index,
                           mask);
            if (!page) {
                btrfs_delalloc_release_metadata(inode,
                            PAGE_CACHE_SIZE);
                ret = -ENOMEM;
                goto out;
            }
        }

        if (PageReadahead(page)) {
            page_cache_async_readahead(inode->i_mapping,
                           ra, NULL, page, index,
                           last_index + 1 - index);
        }

        if (!PageUptodate(page)) {
            btrfs_readpage(NULL, page);
            lock_page(page);
            if (!PageUptodate(page)) {
                unlock_page(page);
                page_cache_release(page);
                btrfs_delalloc_release_metadata(inode,
                            PAGE_CACHE_SIZE);
                ret = -EIO;
                goto out;
            }
        }

        page_start = (u64)page->index << PAGE_CACHE_SHIFT;
        page_end = page_start + PAGE_CACHE_SIZE - 1;

        lock_extent(&BTRFS_I(inode)->io_tree, page_start, page_end);

        set_page_extent_mapped(page);

        if (nr < cluster->nr &&
            page_start + offset == cluster->boundary[nr]) {
            set_extent_bits(&BTRFS_I(inode)->io_tree,
                    page_start, page_end,
                    EXTENT_BOUNDARY, GFP_NOFS);
            nr++;
        }

        btrfs_set_extent_delalloc(inode, page_start, page_end, NULL);
        set_page_dirty(page);

        unlock_extent(&BTRFS_I(inode)->io_tree,
                  page_start, page_end);
        unlock_page(page);
        page_cache_release(page);

        index++;
        balance_dirty_pages_ratelimited(inode->i_mapping);
        btrfs_throttle(BTRFS_I(inode)->root);
    }
    WARN_ON(nr != cluster->nr);
out:
    kfree(ra);
    return ret;
}

static noinline_for_stack
int relocate_data_extent(struct inode *inode, struct btrfs_key *extent_key,
             struct file_extent_cluster *cluster)
{
    int ret;

    if (cluster->nr > 0 && extent_key->objectid != cluster->end + 1) {
        ret = relocate_file_extent_cluster(inode, cluster);
        if (ret)
            return ret;
        cluster->nr = 0;
    }

    if (!cluster->nr)
        cluster->start = extent_key->objectid;
    else
        BUG_ON(cluster->nr >= MAX_EXTENTS);
    cluster->end = extent_key->objectid + extent_key->offset - 1;
    cluster->boundary[cluster->nr] = extent_key->objectid;
    cluster->nr++;

    if (cluster->nr >= MAX_EXTENTS) {
        ret = relocate_file_extent_cluster(inode, cluster);
        if (ret)
            return ret;
        cluster->nr = 0;
    }
    return 0;
}

#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
static int get_ref_objectid_v0(struct reloc_control *rc,
                   struct btrfs_path *path,
                   struct btrfs_key *extent_key,
                   u64 *ref_objectid, int *path_change)
{
    struct btrfs_key key;
    struct extent_buffer *leaf;
    struct btrfs_extent_ref_v0 *ref0;
    int ret;
    int slot;

    leaf = path->nodes[0];
    slot = path->slots[0];
    while (1) {
        if (slot >= btrfs_header_nritems(leaf)) {
            ret = btrfs_next_leaf(rc->extent_root, path);
            if (ret < 0)
                return ret;
            BUG_ON(ret > 0);
            leaf = path->nodes[0];
            slot = path->slots[0];
            if (path_change)
                *path_change = 1;
        }
        btrfs_item_key_to_cpu(leaf, &key, slot);
        if (key.objectid != extent_key->objectid)
            return -ENOENT;

        if (key.type != BTRFS_EXTENT_REF_V0_KEY) {
            slot++;
            continue;
        }
        ref0 = btrfs_item_ptr(leaf, slot,
                struct btrfs_extent_ref_v0);
        *ref_objectid = btrfs_ref_objectid_v0(leaf, ref0);
        break;
    }
    return 0;
}
#endif

/*
 * helper to add a tree block to the list.
 * the major work is getting the generation and level of the block
 */
static int add_tree_block(struct reloc_control *rc,
              struct btrfs_key *extent_key,
              struct btrfs_path *path,
              struct rb_root *blocks)
{
    struct extent_buffer *eb;
    struct btrfs_extent_item *ei;
    struct btrfs_tree_block_info *bi;
    struct tree_block *block;
    struct rb_node *rb_node;
    u32 item_size;
    int level = -1;
    int generation;

    eb =  path->nodes[0];
    item_size = btrfs_item_size_nr(eb, path->slots[0]);

    if (item_size >= sizeof(*ei) + sizeof(*bi)) {
        ei = btrfs_item_ptr(eb, path->slots[0],
                struct btrfs_extent_item);
        bi = (struct btrfs_tree_block_info *)(ei + 1);
        generation = btrfs_extent_generation(eb, ei);
        level = btrfs_tree_block_level(eb, bi);
    } else {
#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
        u64 ref_owner;
        int ret;

        BUG_ON(item_size != sizeof(struct btrfs_extent_item_v0));
        ret = get_ref_objectid_v0(rc, path, extent_key,
                      &ref_owner, NULL);
        if (ret < 0)
            return ret;
        BUG_ON(ref_owner >= BTRFS_MAX_LEVEL);
        level = (int)ref_owner;
        /* FIXME: get real generation */
        generation = 0;
#else
        BUG();
#endif
    }

    btrfs_release_path(path);

    BUG_ON(level == -1);

    block = kmalloc(sizeof(*block), GFP_NOFS);
    if (!block)
        return -ENOMEM;

    block->bytenr = extent_key->objectid;
    block->key.objectid = extent_key->offset;
    block->key.offset = generation;
    block->level = level;
    block->key_ready = 0;

    rb_node = tree_insert(blocks, block->bytenr, &block->rb_node);
    if (rb_node)
        backref_tree_panic(rb_node, -EEXIST, block->bytenr);

    return 0;
}

/*
 * helper to add tree blocks for backref of type BTRFS_SHARED_DATA_REF_KEY
 */
static int __add_tree_block(struct reloc_control *rc,
                u64 bytenr, u32 blocksize,
                struct rb_root *blocks)
{
    struct btrfs_path *path;
    struct btrfs_key key;
    int ret;

    if (tree_block_processed(bytenr, blocksize, rc))
        return 0;

    if (tree_search(blocks, bytenr))
        return 0;

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;

    key.objectid = bytenr;
    key.type = BTRFS_EXTENT_ITEM_KEY;
    key.offset = blocksize;

    path->search_commit_root = 1;
    path->skip_locking = 1;
    ret = btrfs_search_slot(NULL, rc->extent_root, &key, path, 0, 0);
    if (ret < 0)
        goto out;
    BUG_ON(ret);

    btrfs_item_key_to_cpu(path->nodes[0], &key, path->slots[0]);
    ret = add_tree_block(rc, &key, path, blocks);
out:
    btrfs_free_path(path);
    return ret;
}

/*
 * helper to check if the block use full backrefs for pointers in it
 */
static int block_use_full_backref(struct reloc_control *rc,
                  struct extent_buffer *eb)
{
    u64 flags;
    int ret;

    if (btrfs_header_flag(eb, BTRFS_HEADER_FLAG_RELOC) ||
        btrfs_header_backref_rev(eb) < BTRFS_MIXED_BACKREF_REV)
        return 1;

    ret = btrfs_lookup_extent_info(NULL, rc->extent_root,
                       eb->start, eb->len, NULL, &flags);
    BUG_ON(ret);

    if (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF)
        ret = 1;
    else
        ret = 0;
    return ret;
}

static int delete_block_group_cache(struct btrfs_fs_info *fs_info,
                    struct inode *inode, u64 ino)
{
    struct btrfs_key key;
    struct btrfs_path *path;
    struct btrfs_root *root = fs_info->tree_root;
    struct btrfs_trans_handle *trans;
    unsigned long nr;
    int ret = 0;

    if (inode)
        goto truncate;

    key.objectid = ino;
    key.type = BTRFS_INODE_ITEM_KEY;
    key.offset = 0;

    inode = btrfs_iget(fs_info->sb, &key, root, NULL);
    if (IS_ERR(inode) || is_bad_inode(inode)) {
        if (!IS_ERR(inode))
            iput(inode);
        return -ENOENT;
    }

truncate:
    path = btrfs_alloc_path();
    if (!path) {
        ret = -ENOMEM;
        goto out;
    }

    trans = btrfs_join_transaction(root);
    if (IS_ERR(trans)) {
        btrfs_free_path(path);
        ret = PTR_ERR(trans);
        goto out;
    }

    ret = btrfs_truncate_free_space_cache(root, trans, path, inode);

    btrfs_free_path(path);
    nr = trans->blocks_used;
    btrfs_end_transaction(trans, root);
    btrfs_btree_balance_dirty(root, nr);
out:
    iput(inode);
    return ret;
}

/*
 * helper to add tree blocks for backref of type BTRFS_EXTENT_DATA_REF_KEY
 * this function scans fs tree to find blocks reference the data extent
 */
static int find_data_references(struct reloc_control *rc,
                struct btrfs_key *extent_key,
                struct extent_buffer *leaf,
                struct btrfs_extent_data_ref *ref,
                struct rb_root *blocks)
{
    struct btrfs_path *path;
    struct tree_block *block;
    struct btrfs_root *root;
    struct btrfs_file_extent_item *fi;
    struct rb_node *rb_node;
    struct btrfs_key key;
    u64 ref_root;
    u64 ref_objectid;
    u64 ref_offset;
    u32 ref_count;
    u32 nritems;
    int err = 0;
    int added = 0;
    int counted;
    int ret;

    ref_root = btrfs_extent_data_ref_root(leaf, ref);
    ref_objectid = btrfs_extent_data_ref_objectid(leaf, ref);
    ref_offset = btrfs_extent_data_ref_offset(leaf, ref);
    ref_count = btrfs_extent_data_ref_count(leaf, ref);

    /*
     * This is an extent belonging to the free space cache, lets just delete
     * it and redo the search.
     */
    if (ref_root == BTRFS_ROOT_TREE_OBJECTID) {
        ret = delete_block_group_cache(rc->extent_root->fs_info,
                           NULL, ref_objectid);
        if (ret != -ENOENT)
            return ret;
        ret = 0;
    }

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;
    path->reada = 1;

    root = read_fs_root(rc->extent_root->fs_info, ref_root);
    if (IS_ERR(root)) {
        err = PTR_ERR(root);
        goto out;
    }

    key.objectid = ref_objectid;
    key.type = BTRFS_EXTENT_DATA_KEY;
    if (ref_offset > ((u64)-1 << 32))
        key.offset = 0;
    else
        key.offset = ref_offset;

    path->search_commit_root = 1;
    path->skip_locking = 1;
    ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
    if (ret < 0) {
        err = ret;
        goto out;
    }

    leaf = path->nodes[0];
    nritems = btrfs_header_nritems(leaf);
    /*
     * the references in tree blocks that use full backrefs
     * are not counted in
     */
    if (block_use_full_backref(rc, leaf))
        counted = 0;
    else
        counted = 1;
    rb_node = tree_search(blocks, leaf->start);
    if (rb_node) {
        if (counted)
            added = 1;
        else
            path->slots[0] = nritems;
    }

    while (ref_count > 0) {
        while (path->slots[0] >= nritems) {
            ret = btrfs_next_leaf(root, path);
            if (ret < 0) {
                err = ret;
                goto out;
            }
            if (ret > 0) {
                WARN_ON(1);
                goto out;
            }

            leaf = path->nodes[0];
            nritems = btrfs_header_nritems(leaf);
            added = 0;

            if (block_use_full_backref(rc, leaf))
                counted = 0;
            else
                counted = 1;
            rb_node = tree_search(blocks, leaf->start);
            if (rb_node) {
                if (counted)
                    added = 1;
                else
                    path->slots[0] = nritems;
            }
        }

        btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
        if (key.objectid != ref_objectid ||
            key.type != BTRFS_EXTENT_DATA_KEY) {
            WARN_ON(1);
            break;
        }

        fi = btrfs_item_ptr(leaf, path->slots[0],
                    struct btrfs_file_extent_item);

        if (btrfs_file_extent_type(leaf, fi) ==
            BTRFS_FILE_EXTENT_INLINE)
            goto next;

        if (btrfs_file_extent_disk_bytenr(leaf, fi) !=
            extent_key->objectid)
            goto next;

        key.offset -= btrfs_file_extent_offset(leaf, fi);
        if (key.offset != ref_offset)
            goto next;

        if (counted)
            ref_count--;
        if (added)
            goto next;

        if (!tree_block_processed(leaf->start, leaf->len, rc)) {
            block = kmalloc(sizeof(*block), GFP_NOFS);
            if (!block) {
                err = -ENOMEM;
                break;
            }
            block->bytenr = leaf->start;
            btrfs_item_key_to_cpu(leaf, &block->key, 0);
            block->level = 0;
            block->key_ready = 1;
            rb_node = tree_insert(blocks, block->bytenr,
                          &block->rb_node);
            if (rb_node)
                backref_tree_panic(rb_node, -EEXIST,
                           block->bytenr);
        }
        if (counted)
            added = 1;
        else
            path->slots[0] = nritems;
next:
        path->slots[0]++;

    }
out:
    btrfs_free_path(path);
    return err;
}

/*
 * hepler to find all tree blocks that reference a given data extent
 */
static noinline_for_stack
int add_data_references(struct reloc_control *rc,
            struct btrfs_key *extent_key,
            struct btrfs_path *path,
            struct rb_root *blocks)
{
    struct btrfs_key key;
    struct extent_buffer *eb;
    struct btrfs_extent_data_ref *dref;
    struct btrfs_extent_inline_ref *iref;
    unsigned long ptr;
    unsigned long end;
    u32 blocksize = btrfs_level_size(rc->extent_root, 0);
    int ret;
    int err = 0;

    eb = path->nodes[0];
    ptr = btrfs_item_ptr_offset(eb, path->slots[0]);
    end = ptr + btrfs_item_size_nr(eb, path->slots[0]);
#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
    if (ptr + sizeof(struct btrfs_extent_item_v0) == end)
        ptr = end;
    else
#endif
        ptr += sizeof(struct btrfs_extent_item);

    while (ptr < end) {
        iref = (struct btrfs_extent_inline_ref *)ptr;
        key.type = btrfs_extent_inline_ref_type(eb, iref);
        if (key.type == BTRFS_SHARED_DATA_REF_KEY) {
            key.offset = btrfs_extent_inline_ref_offset(eb, iref);
            ret = __add_tree_block(rc, key.offset, blocksize,
                           blocks);
        } else if (key.type == BTRFS_EXTENT_DATA_REF_KEY) {
            dref = (struct btrfs_extent_data_ref *)(&iref->offset);
            ret = find_data_references(rc, extent_key,
                           eb, dref, blocks);
        } else {
            BUG();
        }
        ptr += btrfs_extent_inline_ref_size(key.type);
    }
    WARN_ON(ptr > end);

    while (1) {
        cond_resched();
        eb = path->nodes[0];
        if (path->slots[0] >= btrfs_header_nritems(eb)) {
            ret = btrfs_next_leaf(rc->extent_root, path);
            if (ret < 0) {
                err = ret;
                break;
            }
            if (ret > 0)
                break;
            eb = path->nodes[0];
        }

        btrfs_item_key_to_cpu(eb, &key, path->slots[0]);
        if (key.objectid != extent_key->objectid)
            break;

#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
        if (key.type == BTRFS_SHARED_DATA_REF_KEY ||
            key.type == BTRFS_EXTENT_REF_V0_KEY) {
#else
        BUG_ON(key.type == BTRFS_EXTENT_REF_V0_KEY);
        if (key.type == BTRFS_SHARED_DATA_REF_KEY) {
#endif
            ret = __add_tree_block(rc, key.offset, blocksize,
                           blocks);
        } else if (key.type == BTRFS_EXTENT_DATA_REF_KEY) {
            dref = btrfs_item_ptr(eb, path->slots[0],
                          struct btrfs_extent_data_ref);
            ret = find_data_references(rc, extent_key,
                           eb, dref, blocks);
        } else {
            ret = 0;
        }
        if (ret) {
            err = ret;
            break;
        }
        path->slots[0]++;
    }
    btrfs_release_path(path);
    if (err)
        free_block_list(blocks);
    return err;
}

/*
 * hepler to find next unprocessed extent
 */
static noinline_for_stack
int find_next_extent(struct btrfs_trans_handle *trans,
             struct reloc_control *rc, struct btrfs_path *path,
             struct btrfs_key *extent_key)
{
    struct btrfs_key key;
    struct extent_buffer *leaf;
    u64 start, end, last;
    int ret;

    last = rc->block_group->key.objectid + rc->block_group->key.offset;
    while (1) {
        cond_resched();
        if (rc->search_start >= last) {
            ret = 1;
            break;
        }

        key.objectid = rc->search_start;
        key.type = BTRFS_EXTENT_ITEM_KEY;
        key.offset = 0;

        path->search_commit_root = 1;
        path->skip_locking = 1;
        ret = btrfs_search_slot(NULL, rc->extent_root, &key, path,
                    0, 0);
        if (ret < 0)
            break;
next:
        leaf = path->nodes[0];
        if (path->slots[0] >= btrfs_header_nritems(leaf)) {
            ret = btrfs_next_leaf(rc->extent_root, path);
            if (ret != 0)
                break;
            leaf = path->nodes[0];
        }

        btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
        if (key.objectid >= last) {
            ret = 1;
            break;
        }

        if (key.type != BTRFS_EXTENT_ITEM_KEY ||
            key.objectid + key.offset <= rc->search_start) {
            path->slots[0]++;
            goto next;
        }

        ret = find_first_extent_bit(&rc->processed_blocks,
                        key.objectid, &start, &end,
                        EXTENT_DIRTY, NULL);

        if (ret == 0 && start <= key.objectid) {
            btrfs_release_path(path);
            rc->search_start = end + 1;
        } else {
            rc->search_start = key.objectid + key.offset;
            memcpy(extent_key, &key, sizeof(key));
            return 0;
        }
    }
    btrfs_release_path(path);
    return ret;
}

static void set_reloc_control(struct reloc_control *rc)
{
    struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;

    mutex_lock(&fs_info->reloc_mutex);
    fs_info->reloc_ctl = rc;
    mutex_unlock(&fs_info->reloc_mutex);
}

static void unset_reloc_control(struct reloc_control *rc)
{
    struct btrfs_fs_info *fs_info = rc->extent_root->fs_info;

    mutex_lock(&fs_info->reloc_mutex);
    fs_info->reloc_ctl = NULL;
    mutex_unlock(&fs_info->reloc_mutex);
}

static int check_extent_flags(u64 flags)
{
    if ((flags & BTRFS_EXTENT_FLAG_DATA) &&
        (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK))
        return 1;
    if (!(flags & BTRFS_EXTENT_FLAG_DATA) &&
        !(flags & BTRFS_EXTENT_FLAG_TREE_BLOCK))
        return 1;
    if ((flags & BTRFS_EXTENT_FLAG_DATA) &&
        (flags & BTRFS_BLOCK_FLAG_FULL_BACKREF))
        return 1;
    return 0;
}

static noinline_for_stack
int prepare_to_relocate(struct reloc_control *rc)
{
    struct btrfs_trans_handle *trans;
    int ret;

    rc->block_rsv = btrfs_alloc_block_rsv(rc->extent_root,
                          BTRFS_BLOCK_RSV_TEMP);
    if (!rc->block_rsv)
        return -ENOMEM;

    /*
     * reserve some space for creating reloc trees.
     * btrfs_init_reloc_root will use them when there
     * is no reservation in transaction handle.
     */
    ret = btrfs_block_rsv_add(rc->extent_root, rc->block_rsv,
                  rc->extent_root->nodesize * 256);
    if (ret)
        return ret;

    memset(&rc->cluster, 0, sizeof(rc->cluster));
    rc->search_start = rc->block_group->key.objectid;
    rc->extents_found = 0;
    rc->nodes_relocated = 0;
    rc->merging_rsv_size = 0;

    rc->create_reloc_tree = 1;
    set_reloc_control(rc);

    trans = btrfs_join_transaction(rc->extent_root);
    BUG_ON(IS_ERR(trans));
    btrfs_commit_transaction(trans, rc->extent_root);
    return 0;
}

static noinline_for_stack int relocate_block_group(struct reloc_control *rc)
{
    struct rb_root blocks = RB_ROOT;
    struct btrfs_key key;
    struct btrfs_trans_handle *trans = NULL;
    struct btrfs_path *path;
    struct btrfs_extent_item *ei;
    unsigned long nr;
    u64 flags;
    u32 item_size;
    int ret;
    int err = 0;
    int progress = 0;

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;
    path->reada = 1;

    ret = prepare_to_relocate(rc);
    if (ret) {
        err = ret;
        goto out_free;
    }

    while (1) {
        progress++;
        trans = btrfs_start_transaction(rc->extent_root, 0);
        BUG_ON(IS_ERR(trans));
restart:
        if (update_backref_cache(trans, &rc->backref_cache)) {
            btrfs_end_transaction(trans, rc->extent_root);
            continue;
        }

        ret = find_next_extent(trans, rc, path, &key);
        if (ret < 0)
            err = ret;
        if (ret != 0)
            break;

        rc->extents_found++;

        ei = btrfs_item_ptr(path->nodes[0], path->slots[0],
                    struct btrfs_extent_item);
        item_size = btrfs_item_size_nr(path->nodes[0], path->slots[0]);
        if (item_size >= sizeof(*ei)) {
            flags = btrfs_extent_flags(path->nodes[0], ei);
            ret = check_extent_flags(flags);
            BUG_ON(ret);

        } else {
#ifdef BTRFS_COMPAT_EXTENT_TREE_V0
            u64 ref_owner;
            int path_change = 0;

            BUG_ON(item_size !=
                   sizeof(struct btrfs_extent_item_v0));
            ret = get_ref_objectid_v0(rc, path, &key, &ref_owner,
                          &path_change);
            if (ref_owner < BTRFS_FIRST_FREE_OBJECTID)
                flags = BTRFS_EXTENT_FLAG_TREE_BLOCK;
            else
                flags = BTRFS_EXTENT_FLAG_DATA;

            if (path_change) {
                btrfs_release_path(path);

                path->search_commit_root = 1;
                path->skip_locking = 1;
                ret = btrfs_search_slot(NULL, rc->extent_root,
                            &key, path, 0, 0);
                if (ret < 0) {
                    err = ret;
                    break;
                }
                BUG_ON(ret > 0);
            }
#else
            BUG();
#endif
        }

        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
            ret = add_tree_block(rc, &key, path, &blocks);
        } else if (rc->stage == UPDATE_DATA_PTRS &&
               (flags & BTRFS_EXTENT_FLAG_DATA)) {
            ret = add_data_references(rc, &key, path, &blocks);
        } else {
            btrfs_release_path(path);
            ret = 0;
        }
        if (ret < 0) {
            err = ret;
            break;
        }

        if (!RB_EMPTY_ROOT(&blocks)) {
            ret = relocate_tree_blocks(trans, rc, &blocks);
            if (ret < 0) {
                if (ret != -EAGAIN) {
                    err = ret;
                    break;
                }
                rc->extents_found--;
                rc->search_start = key.objectid;
            }
        }

        ret = btrfs_block_rsv_check(rc->extent_root, rc->block_rsv, 5);
        if (ret < 0) {
            if (ret != -ENOSPC) {
                err = ret;
                WARN_ON(1);
                break;
            }
            rc->commit_transaction = 1;
        }

        if (rc->commit_transaction) {
            rc->commit_transaction = 0;
            ret = btrfs_commit_transaction(trans, rc->extent_root);
            BUG_ON(ret);
        } else {
            nr = trans->blocks_used;
            btrfs_end_transaction_throttle(trans, rc->extent_root);
            btrfs_btree_balance_dirty(rc->extent_root, nr);
        }
        trans = NULL;

        if (rc->stage == MOVE_DATA_EXTENTS &&
            (flags & BTRFS_EXTENT_FLAG_DATA)) {
            rc->found_file_extent = 1;
            ret = relocate_data_extent(rc->data_inode,
                           &key, &rc->cluster);
            if (ret < 0) {
                err = ret;
                break;
            }
        }
    }
    if (trans && progress && err == -ENOSPC) {
        ret = btrfs_force_chunk_alloc(trans, rc->extent_root,
                          rc->block_group->flags);
        if (ret == 0) {
            err = 0;
            progress = 0;
            goto restart;
        }
    }

    btrfs_release_path(path);
    clear_extent_bits(&rc->processed_blocks, 0, (u64)-1, EXTENT_DIRTY,
              GFP_NOFS);

    if (trans) {
        nr = trans->blocks_used;
        btrfs_end_transaction_throttle(trans, rc->extent_root);
        btrfs_btree_balance_dirty(rc->extent_root, nr);
    }

    if (!err) {
        ret = relocate_file_extent_cluster(rc->data_inode,
                           &rc->cluster);
        if (ret < 0)
            err = ret;
    }

    rc->create_reloc_tree = 0;
    set_reloc_control(rc);

    backref_cache_cleanup(&rc->backref_cache);
    btrfs_block_rsv_release(rc->extent_root, rc->block_rsv, (u64)-1);

    err = prepare_to_merge(rc, err);

    merge_reloc_roots(rc);

    rc->merge_reloc_tree = 0;
    unset_reloc_control(rc);
    btrfs_block_rsv_release(rc->extent_root, rc->block_rsv, (u64)-1);

    /* get rid of pinned extents */
    trans = btrfs_join_transaction(rc->extent_root);
    if (IS_ERR(trans))
        err = PTR_ERR(trans);
    else
        btrfs_commit_transaction(trans, rc->extent_root);
out_free:
    btrfs_free_block_rsv(rc->extent_root, rc->block_rsv);
    btrfs_free_path(path);
    return err;
}

static int __insert_orphan_inode(struct btrfs_trans_handle *trans,
                 struct btrfs_root *root, u64 objectid)
{
    struct btrfs_path *path;
    struct btrfs_inode_item *item;
    struct extent_buffer *leaf;
    int ret;

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;

    ret = btrfs_insert_empty_inode(trans, root, path, objectid);
    if (ret)
        goto out;

    leaf = path->nodes[0];
    item = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_inode_item);
    memset_extent_buffer(leaf, 0, (unsigned long)item, sizeof(*item));
    btrfs_set_inode_generation(leaf, item, 1);
    btrfs_set_inode_size(leaf, item, 0);
    btrfs_set_inode_mode(leaf, item, S_IFREG | 0600);
    btrfs_set_inode_flags(leaf, item, BTRFS_INODE_NOCOMPRESS |
                      BTRFS_INODE_PREALLOC);
    btrfs_mark_buffer_dirty(leaf);
    btrfs_release_path(path);
out:
    btrfs_free_path(path);
    return ret;
}

/*
 * helper to create inode for data relocation.
 * the inode is in data relocation tree and its link count is 0
 */
static noinline_for_stack
struct inode *create_reloc_inode(struct btrfs_fs_info *fs_info,
                 struct btrfs_block_group_cache *group)
{
    struct inode *inode = NULL;
    struct btrfs_trans_handle *trans;
    struct btrfs_root *root;
    struct btrfs_key key;
    unsigned long nr;
    u64 objectid = BTRFS_FIRST_FREE_OBJECTID;
    int err = 0;

    root = read_fs_root(fs_info, BTRFS_DATA_RELOC_TREE_OBJECTID);
    if (IS_ERR(root))
        return ERR_CAST(root);

    trans = btrfs_start_transaction(root, 6);
    if (IS_ERR(trans))
        return ERR_CAST(trans);

    err = btrfs_find_free_objectid(root, &objectid);
    if (err)
        goto out;

    err = __insert_orphan_inode(trans, root, objectid);
    BUG_ON(err);

    key.objectid = objectid;
    key.type = BTRFS_INODE_ITEM_KEY;
    key.offset = 0;
    inode = btrfs_iget(root->fs_info->sb, &key, root, NULL);
    BUG_ON(IS_ERR(inode) || is_bad_inode(inode));
    BTRFS_I(inode)->index_cnt = group->key.objectid;

    err = btrfs_orphan_add(trans, inode);
out:
    nr = trans->blocks_used;
    btrfs_end_transaction(trans, root);
    btrfs_btree_balance_dirty(root, nr);
    if (err) {
        if (inode)
            iput(inode);
        inode = ERR_PTR(err);
    }
    return inode;
}

static struct reloc_control *alloc_reloc_control(void)
{
    struct reloc_control *rc;

    rc = kzalloc(sizeof(*rc), GFP_NOFS);
    if (!rc)
        return NULL;

    INIT_LIST_HEAD(&rc->reloc_roots);
    backref_cache_init(&rc->backref_cache);
    mapping_tree_init(&rc->reloc_root_tree);
    extent_io_tree_init(&rc->processed_blocks, NULL);
    return rc;
}

/*
 * function to relocate all extents in a block group.
 */
int btrfs_relocate_block_group(struct btrfs_root *extent_root, u64 group_start)
{
    struct btrfs_fs_info *fs_info = extent_root->fs_info;
    struct reloc_control *rc;
    struct inode *inode;
    struct btrfs_path *path;
    int ret;
    int rw = 0;
    int err = 0;

    rc = alloc_reloc_control();
    if (!rc)
        return -ENOMEM;

    rc->extent_root = extent_root;

    rc->block_group = btrfs_lookup_block_group(fs_info, group_start);
    BUG_ON(!rc->block_group);

    if (!rc->block_group->ro) {
        ret = btrfs_set_block_group_ro(extent_root, rc->block_group);
        if (ret) {
            err = ret;
            goto out;
        }
        rw = 1;
    }

    path = btrfs_alloc_path();
    if (!path) {
        err = -ENOMEM;
        goto out;
    }

    inode = lookup_free_space_inode(fs_info->tree_root, rc->block_group,
                    path);
    btrfs_free_path(path);

    if (!IS_ERR(inode))
        ret = delete_block_group_cache(fs_info, inode, 0);
    else
        ret = PTR_ERR(inode);

    if (ret && ret != -ENOENT) {
        err = ret;
        goto out;
    }

    rc->data_inode = create_reloc_inode(fs_info, rc->block_group);
    if (IS_ERR(rc->data_inode)) {
        err = PTR_ERR(rc->data_inode);
        rc->data_inode = NULL;
        goto out;
    }

    printk(KERN_INFO "btrfs: relocating block group %llu flags %llu\n",
           (unsigned long long)rc->block_group->key.objectid,
           (unsigned long long)rc->block_group->flags);

    btrfs_start_delalloc_inodes(fs_info->tree_root, 0);
    btrfs_wait_ordered_extents(fs_info->tree_root, 0);

    while (1) {
        mutex_lock(&fs_info->cleaner_mutex);

        btrfs_clean_old_snapshots(fs_info->tree_root);
        ret = relocate_block_group(rc);

        mutex_unlock(&fs_info->cleaner_mutex);
        if (ret < 0) {
            err = ret;
            goto out;
        }

        if (rc->extents_found == 0)
            break;

        printk(KERN_INFO "btrfs: found %llu extents\n",
            (unsigned long long)rc->extents_found);

        if (rc->stage == MOVE_DATA_EXTENTS && rc->found_file_extent) {
            btrfs_wait_ordered_range(rc->data_inode, 0, (u64)-1);
            invalidate_mapping_pages(rc->data_inode->i_mapping,
                         0, -1);
            rc->stage = UPDATE_DATA_PTRS;
        }
    }

    filemap_write_and_wait_range(fs_info->btree_inode->i_mapping,
                     rc->block_group->key.objectid,
                     rc->block_group->key.objectid +
                     rc->block_group->key.offset - 1);

    WARN_ON(rc->block_group->pinned > 0);
    WARN_ON(rc->block_group->reserved > 0);
    WARN_ON(btrfs_block_group_used(&rc->block_group->item) > 0);
out:
    if (err && rw)
        btrfs_set_block_group_rw(extent_root, rc->block_group);
    iput(rc->data_inode);
    btrfs_put_block_group(rc->block_group);
    kfree(rc);
    return err;
}

static noinline_for_stack int mark_garbage_root(struct btrfs_root *root)
{
    struct btrfs_trans_handle *trans;
    int ret, err;

    trans = btrfs_start_transaction(root->fs_info->tree_root, 0);
    if (IS_ERR(trans))
        return PTR_ERR(trans);

    memset(&root->root_item.drop_progress, 0,
        sizeof(root->root_item.drop_progress));
    root->root_item.drop_level = 0;
    btrfs_set_root_refs(&root->root_item, 0);
    ret = btrfs_update_root(trans, root->fs_info->tree_root,
                &root->root_key, &root->root_item);

    err = btrfs_end_transaction(trans, root->fs_info->tree_root);
    if (err)
        return err;
    return ret;
}

/*
 * recover relocation interrupted by system crash.
 *
 * this function resumes merging reloc trees with corresponding fs trees.
 * this is important for keeping the sharing of tree blocks
 */
int btrfs_recover_relocation(struct btrfs_root *root)
{
    LIST_HEAD(reloc_roots);
    struct btrfs_key key;
    struct btrfs_root *fs_root;
    struct btrfs_root *reloc_root;
    struct btrfs_path *path;
    struct extent_buffer *leaf;
    struct reloc_control *rc = NULL;
    struct btrfs_trans_handle *trans;
    int ret;
    int err = 0;

    path = btrfs_alloc_path();
    if (!path)
        return -ENOMEM;
    path->reada = -1;

    key.objectid = BTRFS_TREE_RELOC_OBJECTID;
    key.type = BTRFS_ROOT_ITEM_KEY;
    key.offset = (u64)-1;

    while (1) {
        ret = btrfs_search_slot(NULL, root->fs_info->tree_root, &key,
                    path, 0, 0);
        if (ret < 0) {
            err = ret;
            goto out;
        }
        if (ret > 0) {
            if (path->slots[0] == 0)
                break;
            path->slots[0]--;
        }
        leaf = path->nodes[0];
        btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
        btrfs_release_path(path);

        if (key.objectid != BTRFS_TREE_RELOC_OBJECTID ||
            key.type != BTRFS_ROOT_ITEM_KEY)
            break;

        reloc_root = btrfs_read_fs_root_no_radix(root, &key);
        if (IS_ERR(reloc_root)) {
            err = PTR_ERR(reloc_root);
            goto out;
        }

        list_add(&reloc_root->root_list, &reloc_roots);

        if (btrfs_root_refs(&reloc_root->root_item) > 0) {
            fs_root = read_fs_root(root->fs_info,
                           reloc_root->root_key.offset);
            if (IS_ERR(fs_root)) {
                ret = PTR_ERR(fs_root);
                if (ret != -ENOENT) {
                    err = ret;
                    goto out;
                }
                ret = mark_garbage_root(reloc_root);
                if (ret < 0) {
                    err = ret;
                    goto out;
                }
            }
        }

        if (key.offset == 0)
            break;

        key.offset--;
    }
    btrfs_release_path(path);

    if (list_empty(&reloc_roots))
        goto out;

    rc = alloc_reloc_control();
    if (!rc) {
        err = -ENOMEM;
        goto out;
    }

    rc->extent_root = root->fs_info->extent_root;

    set_reloc_control(rc);

    trans = btrfs_join_transaction(rc->extent_root);
    if (IS_ERR(trans)) {
        unset_reloc_control(rc);
        err = PTR_ERR(trans);
        goto out_free;
    }

    rc->merge_reloc_tree = 1;

    while (!list_empty(&reloc_roots)) {
        reloc_root = list_entry(reloc_roots.next,
                    struct btrfs_root, root_list);
        list_del(&reloc_root->root_list);

        if (btrfs_root_refs(&reloc_root->root_item) == 0) {
            list_add_tail(&reloc_root->root_list,
                      &rc->reloc_roots);
            continue;
        }

        fs_root = read_fs_root(root->fs_info,
                       reloc_root->root_key.offset);
        if (IS_ERR(fs_root)) {
            err = PTR_ERR(fs_root);
            goto out_free;
        }

        err = __add_reloc_root(reloc_root);
        BUG_ON(err < 0); /* -ENOMEM or logic error */
        fs_root->reloc_root = reloc_root;
    }

    err = btrfs_commit_transaction(trans, rc->extent_root);
    if (err)
        goto out_free;

    merge_reloc_roots(rc);

    unset_reloc_control(rc);

    trans = btrfs_join_transaction(rc->extent_root);
    if (IS_ERR(trans))
        err = PTR_ERR(trans);
    else
        err = btrfs_commit_transaction(trans, rc->extent_root);
out_free:
    kfree(rc);
out:
    while (!list_empty(&reloc_roots)) {
        reloc_root = list_entry(reloc_roots.next,
                    struct btrfs_root, root_list);
        list_del(&reloc_root->root_list);
        free_extent_buffer(reloc_root->node);
        free_extent_buffer(reloc_root->commit_root);
        kfree(reloc_root);
    }
    btrfs_free_path(path);

    if (err == 0) {
        /* cleanup orphan inode in data relocation tree */
        fs_root = read_fs_root(root->fs_info,
                       BTRFS_DATA_RELOC_TREE_OBJECTID);
        if (IS_ERR(fs_root))
            err = PTR_ERR(fs_root);
        else
            err = btrfs_orphan_cleanup(fs_root);
    }
    return err;
}

/*
 * helper to add ordered checksum for data relocation.
 *
 * cloning checksum properly handles the nodatasum extents.
 * it also saves CPU time to re-calculate the checksum.
 */
int btrfs_reloc_clone_csums(struct inode *inode, u64 file_pos, u64 len)
{
    struct btrfs_ordered_sum *sums;
    struct btrfs_sector_sum *sector_sum;
    struct btrfs_ordered_extent *ordered;
    struct btrfs_root *root = BTRFS_I(inode)->root;
    size_t offset;
    int ret;
    u64 disk_bytenr;
    LIST_HEAD(list);

    ordered = btrfs_lookup_ordered_extent(inode, file_pos);
    BUG_ON(ordered->file_offset != file_pos || ordered->len != len);

    disk_bytenr = file_pos + BTRFS_I(inode)->index_cnt;
    ret = btrfs_lookup_csums_range(root->fs_info->csum_root, disk_bytenr,
                       disk_bytenr + len - 1, &list, 0);
    if (ret)
        goto out;

    while (!list_empty(&list)) {
        sums = list_entry(list.next, struct btrfs_ordered_sum, list);
        list_del_init(&sums->list);

        sector_sum = sums->sums;
        sums->bytenr = ordered->start;

        offset = 0;
        while (offset < sums->len) {
            sector_sum->bytenr += ordered->start - disk_bytenr;
            sector_sum++;
            offset += root->sectorsize;
        }

        btrfs_add_ordered_sum(inode, ordered, sums);
    }
out:
    btrfs_put_ordered_extent(ordered);
    return ret;
}

void btrfs_reloc_cow_block(struct btrfs_trans_handle *trans,
               struct btrfs_root *root, struct extent_buffer *buf,
               struct extent_buffer *cow)
{
    struct reloc_control *rc;
    struct backref_node *node;
    int first_cow = 0;
    int level;
    int ret;

    rc = root->fs_info->reloc_ctl;
    if (!rc)
        return;

    BUG_ON(rc->stage == UPDATE_DATA_PTRS &&
           root->root_key.objectid == BTRFS_DATA_RELOC_TREE_OBJECTID);

    level = btrfs_header_level(buf);
    if (btrfs_header_generation(buf) <=
        btrfs_root_last_snapshot(&root->root_item))
        first_cow = 1;

    if (root->root_key.objectid == BTRFS_TREE_RELOC_OBJECTID &&
        rc->create_reloc_tree) {
        WARN_ON(!first_cow && level == 0);

        node = rc->backref_cache.path[level];
        BUG_ON(node->bytenr != buf->start &&
               node->new_bytenr != buf->start);

        drop_node_buffer(node);
        extent_buffer_get(cow);
        node->eb = cow;
        node->new_bytenr = cow->start;

        if (!node->pending) {
            list_move_tail(&node->list,
                       &rc->backref_cache.pending[level]);
            node->pending = 1;
        }

        if (first_cow)
            __mark_block_processed(rc, node);

        if (first_cow && level > 0)
            rc->nodes_relocated += buf->len;
    }

    if (level == 0 && first_cow && rc->stage == UPDATE_DATA_PTRS) {
        ret = replace_file_extents(trans, rc, root, cow);
        BUG_ON(ret);
    }
}

/*
 * called before creating snapshot. it calculates metadata reservation
 * requried for relocating tree blocks in the snapshot
 */
void btrfs_reloc_pre_snapshot(struct btrfs_trans_handle *trans,
                  struct btrfs_pending_snapshot *pending,
                  u64 *bytes_to_reserve)
{
    struct btrfs_root *root;
    struct reloc_control *rc;

    root = pending->root;
    if (!root->reloc_root)
        return;

    rc = root->fs_info->reloc_ctl;
    if (!rc->merge_reloc_tree)
        return;

    root = root->reloc_root;
    BUG_ON(btrfs_root_refs(&root->root_item) == 0);
    /*
     * relocation is in the stage of merging trees. the space
     * used by merging a reloc tree is twice the size of
     * relocated tree nodes in the worst case. half for cowing
     * the reloc tree, half for cowing the fs tree. the space
     * used by cowing the reloc tree will be freed after the
     * tree is dropped. if we create snapshot, cowing the fs
     * tree may use more space than it frees. so we need
     * reserve extra space.
     */
    *bytes_to_reserve += rc->nodes_relocated;
}

/*
 * called after snapshot is created. migrate block reservation
 * and create reloc root for the newly created snapshot
 */
int btrfs_reloc_post_snapshot(struct btrfs_trans_handle *trans,
                   struct btrfs_pending_snapshot *pending)
{
    struct btrfs_root *root = pending->root;
    struct btrfs_root *reloc_root;
    struct btrfs_root *new_root;
    struct reloc_control *rc;
    int ret;

    if (!root->reloc_root)
        return 0;

    rc = root->fs_info->reloc_ctl;
    rc->merging_rsv_size += rc->nodes_relocated;

    if (rc->merge_reloc_tree) {
        ret = btrfs_block_rsv_migrate(&pending->block_rsv,
                          rc->block_rsv,
                          rc->nodes_relocated);
        if (ret)
            return ret;
    }

    new_root = pending->snap;
    reloc_root = create_reloc_root(trans, root->reloc_root,
                       new_root->root_key.objectid);
    if (IS_ERR(reloc_root))
        return PTR_ERR(reloc_root);

    ret = __add_reloc_root(reloc_root);
    BUG_ON(ret < 0);
    new_root->reloc_root = reloc_root;

    if (rc->create_reloc_tree)
        ret = clone_backref_node(trans, rc, root, reloc_root);
    return ret;
}