backref.c

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/*
 * Copyright (C) 2011 STRATO.  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/vmalloc.h>
#include "ctree.h"
#include "disk-io.h"
#include "backref.h"
#include "ulist.h"
#include "transaction.h"
#include "delayed-ref.h"
#include "locking.h"

struct extent_inode_elem {
    u64 inum;
    u64 offset;
    struct extent_inode_elem *next;
};

static int check_extent_in_eb(struct btrfs_key *key, struct extent_buffer *eb,
                struct btrfs_file_extent_item *fi,
                u64 extent_item_pos,
                struct extent_inode_elem **eie)
{
    u64 data_offset;
    u64 data_len;
    struct extent_inode_elem *e;

    data_offset = btrfs_file_extent_offset(eb, fi);
    data_len = btrfs_file_extent_num_bytes(eb, fi);

    if (extent_item_pos < data_offset ||
        extent_item_pos >= data_offset + data_len)
        return 1;

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

    e->next = *eie;
    e->inum = key->objectid;
    e->offset = key->offset + (extent_item_pos - data_offset);
    *eie = e;

    return 0;
}

static int find_extent_in_eb(struct extent_buffer *eb, u64 wanted_disk_byte,
                u64 extent_item_pos,
                struct extent_inode_elem **eie)
{
    u64 disk_byte;
    struct btrfs_key key;
    struct btrfs_file_extent_item *fi;
    int slot;
    int nritems;
    int extent_type;
    int ret;

    /*
     * from the shared data ref, we only have the leaf but we need
     * the key. thus, we must look into all items and see that we
     * find one (some) with a reference to our extent item.
     */
    nritems = btrfs_header_nritems(eb);
    for (slot = 0; slot < nritems; ++slot) {
        btrfs_item_key_to_cpu(eb, &key, slot);
        if (key.type != BTRFS_EXTENT_DATA_KEY)
            continue;
        fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
        extent_type = btrfs_file_extent_type(eb, fi);
        if (extent_type == BTRFS_FILE_EXTENT_INLINE)
            continue;
        /* don't skip BTRFS_FILE_EXTENT_PREALLOC, we can handle that */
        disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);
        if (disk_byte != wanted_disk_byte)
            continue;

        ret = check_extent_in_eb(&key, eb, fi, extent_item_pos, eie);
        if (ret < 0)
            return ret;
    }

    return 0;
}

/*
 * this structure records all encountered refs on the way up to the root
 */
struct __prelim_ref {
    struct list_head list;
    u64 root_id;
    struct btrfs_key key_for_search;
    int level;
    int count;
    struct extent_inode_elem *inode_list;
    u64 parent;
    u64 wanted_disk_byte;
};

/*
 * the rules for all callers of this function are:
 * - obtaining the parent is the goal
 * - if you add a key, you must know that it is a correct key
 * - if you cannot add the parent or a correct key, then we will look into the
 *   block later to set a correct key
 *
 * delayed refs
 * ============
 *        backref type | shared | indirect | shared | indirect
 * information         |   tree |     tree |   data |     data
 * --------------------+--------+----------+--------+----------
 *      parent logical |    y   |     -    |    -   |     -
 *      key to resolve |    -   |     y    |    y   |     y
 *  tree block logical |    -   |     -    |    -   |     -
 *  root for resolving |    y   |     y    |    y   |     y
 *
 * - column 1:       we've the parent -> done
 * - column 2, 3, 4: we use the key to find the parent
 *
 * on disk refs (inline or keyed)
 * ==============================
 *        backref type | shared | indirect | shared | indirect
 * information         |   tree |     tree |   data |     data
 * --------------------+--------+----------+--------+----------
 *      parent logical |    y   |     -    |    y   |     -
 *      key to resolve |    -   |     -    |    -   |     y
 *  tree block logical |    y   |     y    |    y   |     y
 *  root for resolving |    -   |     y    |    y   |     y
 *
 * - column 1, 3: we've the parent -> done
 * - column 2:    we take the first key from the block to find the parent
 *                (see __add_missing_keys)
 * - column 4:    we use the key to find the parent
 *
 * additional information that's available but not required to find the parent
 * block might help in merging entries to gain some speed.
 */

static int __add_prelim_ref(struct list_head *head, u64 root_id,
                struct btrfs_key *key, int level,
                u64 parent, u64 wanted_disk_byte, int count)
{
    struct __prelim_ref *ref;

    /* in case we're adding delayed refs, we're holding the refs spinlock */
    ref = kmalloc(sizeof(*ref), GFP_ATOMIC);
    if (!ref)
        return -ENOMEM;

    ref->root_id = root_id;
    if (key)
        ref->key_for_search = *key;
    else
        memset(&ref->key_for_search, 0, sizeof(ref->key_for_search));

    ref->inode_list = NULL;
    ref->level = level;
    ref->count = count;
    ref->parent = parent;
    ref->wanted_disk_byte = wanted_disk_byte;
    list_add_tail(&ref->list, head);

    return 0;
}

static int add_all_parents(struct btrfs_root *root, struct btrfs_path *path,
                struct ulist *parents, int level,
                struct btrfs_key *key_for_search, u64 time_seq,
                u64 wanted_disk_byte,
                const u64 *extent_item_pos)
{
    int ret = 0;
    int slot;
    struct extent_buffer *eb;
    struct btrfs_key key;
    struct btrfs_file_extent_item *fi;
    struct extent_inode_elem *eie = NULL;
    u64 disk_byte;

    if (level != 0) {
        eb = path->nodes[level];
        ret = ulist_add(parents, eb->start, 0, GFP_NOFS);
        if (ret < 0)
            return ret;
        return 0;
    }

    /*
     * We normally enter this function with the path already pointing to
     * the first item to check. But sometimes, we may enter it with
     * slot==nritems. In that case, go to the next leaf before we continue.
     */
    if (path->slots[0] >= btrfs_header_nritems(path->nodes[0]))
        ret = btrfs_next_old_leaf(root, path, time_seq);

    while (!ret) {
        eb = path->nodes[0];
        slot = path->slots[0];

        btrfs_item_key_to_cpu(eb, &key, slot);

        if (key.objectid != key_for_search->objectid ||
            key.type != BTRFS_EXTENT_DATA_KEY)
            break;

        fi = btrfs_item_ptr(eb, slot, struct btrfs_file_extent_item);
        disk_byte = btrfs_file_extent_disk_bytenr(eb, fi);

        if (disk_byte == wanted_disk_byte) {
            eie = NULL;
            if (extent_item_pos) {
                ret = check_extent_in_eb(&key, eb, fi,
                        *extent_item_pos,
                        &eie);
                if (ret < 0)
                    break;
            }
            if (!ret) {
                ret = ulist_add(parents, eb->start,
                        (uintptr_t)eie, GFP_NOFS);
                if (ret < 0)
                    break;
                if (!extent_item_pos) {
                    ret = btrfs_next_old_leaf(root, path,
                            time_seq);
                    continue;
                }
            }
        }
        ret = btrfs_next_old_item(root, path, time_seq);
    }

    if (ret > 0)
        ret = 0;
    return ret;
}

/*
 * resolve an indirect backref in the form (root_id, key, level)
 * to a logical address
 */
static int __resolve_indirect_ref(struct btrfs_fs_info *fs_info,
                    int search_commit_root,
                    u64 time_seq,
                    struct __prelim_ref *ref,
                    struct ulist *parents,
                    const u64 *extent_item_pos)
{
    struct btrfs_path *path;
    struct btrfs_root *root;
    struct btrfs_key root_key;
    struct extent_buffer *eb;
    int ret = 0;
    int root_level;
    int level = ref->level;

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

    root_key.objectid = ref->root_id;
    root_key.type = BTRFS_ROOT_ITEM_KEY;
    root_key.offset = (u64)-1;
    root = btrfs_read_fs_root_no_name(fs_info, &root_key);
    if (IS_ERR(root)) {
        ret = PTR_ERR(root);
        goto out;
    }

    root_level = btrfs_old_root_level(root, time_seq);

    if (root_level + 1 == level)
        goto out;

    path->lowest_level = level;
    ret = btrfs_search_old_slot(root, &ref->key_for_search, path, time_seq);
    pr_debug("search slot in root %llu (level %d, ref count %d) returned "
         "%d for key (%llu %u %llu)\n",
         (unsigned long long)ref->root_id, level, ref->count, ret,
         (unsigned long long)ref->key_for_search.objectid,
         ref->key_for_search.type,
         (unsigned long long)ref->key_for_search.offset);
    if (ret < 0)
        goto out;

    eb = path->nodes[level];
    while (!eb) {
        if (!level) {
            WARN_ON(1);
            ret = 1;
            goto out;
        }
        level--;
        eb = path->nodes[level];
    }

    ret = add_all_parents(root, path, parents, level, &ref->key_for_search,
                time_seq, ref->wanted_disk_byte,
                extent_item_pos);
out:
    btrfs_free_path(path);
    return ret;
}

/*
 * resolve all indirect backrefs from the list
 */
static int __resolve_indirect_refs(struct btrfs_fs_info *fs_info,
                   int search_commit_root, u64 time_seq,
                   struct list_head *head,
                   const u64 *extent_item_pos)
{
    int err;
    int ret = 0;
    struct __prelim_ref *ref;
    struct __prelim_ref *ref_safe;
    struct __prelim_ref *new_ref;
    struct ulist *parents;
    struct ulist_node *node;
    struct ulist_iterator uiter;

    parents = ulist_alloc(GFP_NOFS);
    if (!parents)
        return -ENOMEM;

    /*
     * _safe allows us to insert directly after the current item without
     * iterating over the newly inserted items.
     * we're also allowed to re-assign ref during iteration.
     */
    list_for_each_entry_safe(ref, ref_safe, head, list) {
        if (ref->parent)    /* already direct */
            continue;
        if (ref->count == 0)
            continue;
        err = __resolve_indirect_ref(fs_info, search_commit_root,
                         time_seq, ref, parents,
                         extent_item_pos);
        if (err) {
            if (ret == 0)
                ret = err;
            continue;
        }

        /* we put the first parent into the ref at hand */
        ULIST_ITER_INIT(&uiter);
        node = ulist_next(parents, &uiter);
        ref->parent = node ? node->val : 0;
        ref->inode_list = node ?
            (struct extent_inode_elem *)(uintptr_t)node->aux : 0;

        /* additional parents require new refs being added here */
        while ((node = ulist_next(parents, &uiter))) {
            new_ref = kmalloc(sizeof(*new_ref), GFP_NOFS);
            if (!new_ref) {
                ret = -ENOMEM;
                break;
            }
            memcpy(new_ref, ref, sizeof(*ref));
            new_ref->parent = node->val;
            new_ref->inode_list = (struct extent_inode_elem *)
                            (uintptr_t)node->aux;
            list_add(&new_ref->list, &ref->list);
        }
        ulist_reinit(parents);
    }

    ulist_free(parents);
    return ret;
}

static inline int ref_for_same_block(struct __prelim_ref *ref1,
                     struct __prelim_ref *ref2)
{
    if (ref1->level != ref2->level)
        return 0;
    if (ref1->root_id != ref2->root_id)
        return 0;
    if (ref1->key_for_search.type != ref2->key_for_search.type)
        return 0;
    if (ref1->key_for_search.objectid != ref2->key_for_search.objectid)
        return 0;
    if (ref1->key_for_search.offset != ref2->key_for_search.offset)
        return 0;
    if (ref1->parent != ref2->parent)
        return 0;

    return 1;
}

/*
 * read tree blocks and add keys where required.
 */
static int __add_missing_keys(struct btrfs_fs_info *fs_info,
                  struct list_head *head)
{
    struct list_head *pos;
    struct extent_buffer *eb;

    list_for_each(pos, head) {
        struct __prelim_ref *ref;
        ref = list_entry(pos, struct __prelim_ref, list);

        if (ref->parent)
            continue;
        if (ref->key_for_search.type)
            continue;
        BUG_ON(!ref->wanted_disk_byte);
        eb = read_tree_block(fs_info->tree_root, ref->wanted_disk_byte,
                     fs_info->tree_root->leafsize, 0);
        BUG_ON(!eb);
        btrfs_tree_read_lock(eb);
        if (btrfs_header_level(eb) == 0)
            btrfs_item_key_to_cpu(eb, &ref->key_for_search, 0);
        else
            btrfs_node_key_to_cpu(eb, &ref->key_for_search, 0);
        btrfs_tree_read_unlock(eb);
        free_extent_buffer(eb);
    }
    return 0;
}

/*
 * merge two lists of backrefs and adjust counts accordingly
 *
 * mode = 1: merge identical keys, if key is set
 *    FIXME: if we add more keys in __add_prelim_ref, we can merge more here.
 *           additionally, we could even add a key range for the blocks we
 *           looked into to merge even more (-> replace unresolved refs by those
 *           having a parent).
 * mode = 2: merge identical parents
 */
static int __merge_refs(struct list_head *head, int mode)
{
    struct list_head *pos1;

    list_for_each(pos1, head) {
        struct list_head *n2;
        struct list_head *pos2;
        struct __prelim_ref *ref1;

        ref1 = list_entry(pos1, struct __prelim_ref, list);

        for (pos2 = pos1->next, n2 = pos2->next; pos2 != head;
             pos2 = n2, n2 = pos2->next) {
            struct __prelim_ref *ref2;
            struct __prelim_ref *xchg;

            ref2 = list_entry(pos2, struct __prelim_ref, list);

            if (mode == 1) {
                if (!ref_for_same_block(ref1, ref2))
                    continue;
                if (!ref1->parent && ref2->parent) {
                    xchg = ref1;
                    ref1 = ref2;
                    ref2 = xchg;
                }
                ref1->count += ref2->count;
            } else {
                if (ref1->parent != ref2->parent)
                    continue;
                ref1->count += ref2->count;
            }
            list_del(&ref2->list);
            kfree(ref2);
        }

    }
    return 0;
}

/*
 * add all currently queued delayed refs from this head whose seq nr is
 * smaller or equal that seq to the list
 */
static int __add_delayed_refs(struct btrfs_delayed_ref_head *head, u64 seq,
                  struct list_head *prefs)
{
    struct btrfs_delayed_extent_op *extent_op = head->extent_op;
    struct rb_node *n = &head->node.rb_node;
    struct btrfs_key key;
    struct btrfs_key op_key = {0};
    int sgn;
    int ret = 0;

    if (extent_op && extent_op->update_key)
        btrfs_disk_key_to_cpu(&op_key, &extent_op->key);

    while ((n = rb_prev(n))) {
        struct btrfs_delayed_ref_node *node;
        node = rb_entry(n, struct btrfs_delayed_ref_node,
                rb_node);
        if (node->bytenr != head->node.bytenr)
            break;
        WARN_ON(node->is_head);

        if (node->seq > seq)
            continue;

        switch (node->action) {
        case BTRFS_ADD_DELAYED_EXTENT:
        case BTRFS_UPDATE_DELAYED_HEAD:
            WARN_ON(1);
            continue;
        case BTRFS_ADD_DELAYED_REF:
            sgn = 1;
            break;
        case BTRFS_DROP_DELAYED_REF:
            sgn = -1;
            break;
        default:
            BUG_ON(1);
        }
        switch (node->type) {
        case BTRFS_TREE_BLOCK_REF_KEY: {
            struct btrfs_delayed_tree_ref *ref;

            ref = btrfs_delayed_node_to_tree_ref(node);
            ret = __add_prelim_ref(prefs, ref->root, &op_key,
                           ref->level + 1, 0, node->bytenr,
                           node->ref_mod * sgn);
            break;
        }
        case BTRFS_SHARED_BLOCK_REF_KEY: {
            struct btrfs_delayed_tree_ref *ref;

            ref = btrfs_delayed_node_to_tree_ref(node);
            ret = __add_prelim_ref(prefs, ref->root, NULL,
                           ref->level + 1, ref->parent,
                           node->bytenr,
                           node->ref_mod * sgn);
            break;
        }
        case BTRFS_EXTENT_DATA_REF_KEY: {
            struct btrfs_delayed_data_ref *ref;
            ref = btrfs_delayed_node_to_data_ref(node);

            key.objectid = ref->objectid;
            key.type = BTRFS_EXTENT_DATA_KEY;
            key.offset = ref->offset;
            ret = __add_prelim_ref(prefs, ref->root, &key, 0, 0,
                           node->bytenr,
                           node->ref_mod * sgn);
            break;
        }
        case BTRFS_SHARED_DATA_REF_KEY: {
            struct btrfs_delayed_data_ref *ref;

            ref = btrfs_delayed_node_to_data_ref(node);

            key.objectid = ref->objectid;
            key.type = BTRFS_EXTENT_DATA_KEY;
            key.offset = ref->offset;
            ret = __add_prelim_ref(prefs, ref->root, &key, 0,
                           ref->parent, node->bytenr,
                           node->ref_mod * sgn);
            break;
        }
        default:
            WARN_ON(1);
        }
        BUG_ON(ret);
    }

    return 0;
}

/*
 * add all inline backrefs for bytenr to the list
 */
static int __add_inline_refs(struct btrfs_fs_info *fs_info,
                 struct btrfs_path *path, u64 bytenr,
                 int *info_level, struct list_head *prefs)
{
    int ret = 0;
    int slot;
    struct extent_buffer *leaf;
    struct btrfs_key key;
    unsigned long ptr;
    unsigned long end;
    struct btrfs_extent_item *ei;
    u64 flags;
    u64 item_size;

    /*
     * enumerate all inline refs
     */
    leaf = path->nodes[0];
    slot = path->slots[0];

    item_size = btrfs_item_size_nr(leaf, slot);
    BUG_ON(item_size < sizeof(*ei));

    ei = btrfs_item_ptr(leaf, slot, struct btrfs_extent_item);
    flags = btrfs_extent_flags(leaf, ei);

    ptr = (unsigned long)(ei + 1);
    end = (unsigned long)ei + item_size;

    if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
        struct btrfs_tree_block_info *info;

        info = (struct btrfs_tree_block_info *)ptr;
        *info_level = btrfs_tree_block_level(leaf, info);
        ptr += sizeof(struct btrfs_tree_block_info);
        BUG_ON(ptr > end);
    } else {
        BUG_ON(!(flags & BTRFS_EXTENT_FLAG_DATA));
    }

    while (ptr < end) {
        struct btrfs_extent_inline_ref *iref;
        u64 offset;
        int type;

        iref = (struct btrfs_extent_inline_ref *)ptr;
        type = btrfs_extent_inline_ref_type(leaf, iref);
        offset = btrfs_extent_inline_ref_offset(leaf, iref);

        switch (type) {
        case BTRFS_SHARED_BLOCK_REF_KEY:
            ret = __add_prelim_ref(prefs, 0, NULL,
                        *info_level + 1, offset,
                        bytenr, 1);
            break;
        case BTRFS_SHARED_DATA_REF_KEY: {
            struct btrfs_shared_data_ref *sdref;
            int count;

            sdref = (struct btrfs_shared_data_ref *)(iref + 1);
            count = btrfs_shared_data_ref_count(leaf, sdref);
            ret = __add_prelim_ref(prefs, 0, NULL, 0, offset,
                           bytenr, count);
            break;
        }
        case BTRFS_TREE_BLOCK_REF_KEY:
            ret = __add_prelim_ref(prefs, offset, NULL,
                           *info_level + 1, 0,
                           bytenr, 1);
            break;
        case BTRFS_EXTENT_DATA_REF_KEY: {
            struct btrfs_extent_data_ref *dref;
            int count;
            u64 root;

            dref = (struct btrfs_extent_data_ref *)(&iref->offset);
            count = btrfs_extent_data_ref_count(leaf, dref);
            key.objectid = btrfs_extent_data_ref_objectid(leaf,
                                      dref);
            key.type = BTRFS_EXTENT_DATA_KEY;
            key.offset = btrfs_extent_data_ref_offset(leaf, dref);
            root = btrfs_extent_data_ref_root(leaf, dref);
            ret = __add_prelim_ref(prefs, root, &key, 0, 0,
                           bytenr, count);
            break;
        }
        default:
            WARN_ON(1);
        }
        BUG_ON(ret);
        ptr += btrfs_extent_inline_ref_size(type);
    }

    return 0;
}

/*
 * add all non-inline backrefs for bytenr to the list
 */
static int __add_keyed_refs(struct btrfs_fs_info *fs_info,
                struct btrfs_path *path, u64 bytenr,
                int info_level, struct list_head *prefs)
{
    struct btrfs_root *extent_root = fs_info->extent_root;
    int ret;
    int slot;
    struct extent_buffer *leaf;
    struct btrfs_key key;

    while (1) {
        ret = btrfs_next_item(extent_root, path);
        if (ret < 0)
            break;
        if (ret) {
            ret = 0;
            break;
        }

        slot = path->slots[0];
        leaf = path->nodes[0];
        btrfs_item_key_to_cpu(leaf, &key, slot);

        if (key.objectid != bytenr)
            break;
        if (key.type < BTRFS_TREE_BLOCK_REF_KEY)
            continue;
        if (key.type > BTRFS_SHARED_DATA_REF_KEY)
            break;

        switch (key.type) {
        case BTRFS_SHARED_BLOCK_REF_KEY:
            ret = __add_prelim_ref(prefs, 0, NULL,
                        info_level + 1, key.offset,
                        bytenr, 1);
            break;
        case BTRFS_SHARED_DATA_REF_KEY: {
            struct btrfs_shared_data_ref *sdref;
            int count;

            sdref = btrfs_item_ptr(leaf, slot,
                          struct btrfs_shared_data_ref);
            count = btrfs_shared_data_ref_count(leaf, sdref);
            ret = __add_prelim_ref(prefs, 0, NULL, 0, key.offset,
                        bytenr, count);
            break;
        }
        case BTRFS_TREE_BLOCK_REF_KEY:
            ret = __add_prelim_ref(prefs, key.offset, NULL,
                           info_level + 1, 0,
                           bytenr, 1);
            break;
        case BTRFS_EXTENT_DATA_REF_KEY: {
            struct btrfs_extent_data_ref *dref;
            int count;
            u64 root;

            dref = btrfs_item_ptr(leaf, slot,
                          struct btrfs_extent_data_ref);
            count = btrfs_extent_data_ref_count(leaf, dref);
            key.objectid = btrfs_extent_data_ref_objectid(leaf,
                                      dref);
            key.type = BTRFS_EXTENT_DATA_KEY;
            key.offset = btrfs_extent_data_ref_offset(leaf, dref);
            root = btrfs_extent_data_ref_root(leaf, dref);
            ret = __add_prelim_ref(prefs, root, &key, 0, 0,
                           bytenr, count);
            break;
        }
        default:
            WARN_ON(1);
        }
        BUG_ON(ret);
    }

    return ret;
}

/*
 * this adds all existing backrefs (inline backrefs, backrefs and delayed
 * refs) for the given bytenr to the refs list, merges duplicates and resolves
 * indirect refs to their parent bytenr.
 * When roots are found, they're added to the roots list
 *
 * FIXME some caching might speed things up
 */
static int find_parent_nodes(struct btrfs_trans_handle *trans,
                 struct btrfs_fs_info *fs_info, u64 bytenr,
                 u64 time_seq, struct ulist *refs,
                 struct ulist *roots, const u64 *extent_item_pos)
{
    struct btrfs_key key;
    struct btrfs_path *path;
    struct btrfs_delayed_ref_root *delayed_refs = NULL;
    struct btrfs_delayed_ref_head *head;
    int info_level = 0;
    int ret;
    int search_commit_root = (trans == BTRFS_BACKREF_SEARCH_COMMIT_ROOT);
    struct list_head prefs_delayed;
    struct list_head prefs;
    struct __prelim_ref *ref;

    INIT_LIST_HEAD(&prefs);
    INIT_LIST_HEAD(&prefs_delayed);

    key.objectid = bytenr;
    key.type = BTRFS_EXTENT_ITEM_KEY;
    key.offset = (u64)-1;

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

    /*
     * grab both a lock on the path and a lock on the delayed ref head.
     * We need both to get a consistent picture of how the refs look
     * at a specified point in time
     */
again:
    head = NULL;

    ret = btrfs_search_slot(trans, fs_info->extent_root, &key, path, 0, 0);
    if (ret < 0)
        goto out;
    BUG_ON(ret == 0);

    if (trans != BTRFS_BACKREF_SEARCH_COMMIT_ROOT) {
        /*
         * look if there are updates for this ref queued and lock the
         * head
         */
        delayed_refs = &trans->transaction->delayed_refs;
        spin_lock(&delayed_refs->lock);
        head = btrfs_find_delayed_ref_head(trans, bytenr);
        if (head) {
            if (!mutex_trylock(&head->mutex)) {
                atomic_inc(&head->node.refs);
                spin_unlock(&delayed_refs->lock);

                btrfs_release_path(path);

                /*
                 * Mutex was contended, block until it's
                 * released and try again
                 */
                mutex_lock(&head->mutex);
                mutex_unlock(&head->mutex);
                btrfs_put_delayed_ref(&head->node);
                goto again;
            }
            ret = __add_delayed_refs(head, time_seq,
                         &prefs_delayed);
            mutex_unlock(&head->mutex);
            if (ret) {
                spin_unlock(&delayed_refs->lock);
                goto out;
            }
        }
        spin_unlock(&delayed_refs->lock);
    }

    if (path->slots[0]) {
        struct extent_buffer *leaf;
        int slot;

        path->slots[0]--;
        leaf = path->nodes[0];
        slot = path->slots[0];
        btrfs_item_key_to_cpu(leaf, &key, slot);
        if (key.objectid == bytenr &&
            key.type == BTRFS_EXTENT_ITEM_KEY) {
            ret = __add_inline_refs(fs_info, path, bytenr,
                        &info_level, &prefs);
            if (ret)
                goto out;
            ret = __add_keyed_refs(fs_info, path, bytenr,
                           info_level, &prefs);
            if (ret)
                goto out;
        }
    }
    btrfs_release_path(path);

    list_splice_init(&prefs_delayed, &prefs);

    ret = __add_missing_keys(fs_info, &prefs);
    if (ret)
        goto out;

    ret = __merge_refs(&prefs, 1);
    if (ret)
        goto out;

    ret = __resolve_indirect_refs(fs_info, search_commit_root, time_seq,
                      &prefs, extent_item_pos);
    if (ret)
        goto out;

    ret = __merge_refs(&prefs, 2);
    if (ret)
        goto out;

    while (!list_empty(&prefs)) {
        ref = list_first_entry(&prefs, struct __prelim_ref, list);
        list_del(&ref->list);
        if (ref->count < 0)
            WARN_ON(1);
        if (ref->count && ref->root_id && ref->parent == 0) {
            /* no parent == root of tree */
            ret = ulist_add(roots, ref->root_id, 0, GFP_NOFS);
            BUG_ON(ret < 0);
        }
        if (ref->count && ref->parent) {
            struct extent_inode_elem *eie = NULL;
            if (extent_item_pos && !ref->inode_list) {
                u32 bsz;
                struct extent_buffer *eb;
                bsz = btrfs_level_size(fs_info->extent_root,
                            info_level);
                eb = read_tree_block(fs_info->extent_root,
                               ref->parent, bsz, 0);
                BUG_ON(!eb);
                ret = find_extent_in_eb(eb, bytenr,
                            *extent_item_pos, &eie);
                ref->inode_list = eie;
                free_extent_buffer(eb);
            }
            ret = ulist_add_merge(refs, ref->parent,
                          (uintptr_t)ref->inode_list,
                          (u64 *)&eie, GFP_NOFS);
            if (!ret && extent_item_pos) {
                /*
                 * we've recorded that parent, so we must extend
                 * its inode list here
                 */
                BUG_ON(!eie);
                while (eie->next)
                    eie = eie->next;
                eie->next = ref->inode_list;
            }
            BUG_ON(ret < 0);
        }
        kfree(ref);
    }

out:
    btrfs_free_path(path);
    while (!list_empty(&prefs)) {
        ref = list_first_entry(&prefs, struct __prelim_ref, list);
        list_del(&ref->list);
        kfree(ref);
    }
    while (!list_empty(&prefs_delayed)) {
        ref = list_first_entry(&prefs_delayed, struct __prelim_ref,
                       list);
        list_del(&ref->list);
        kfree(ref);
    }

    return ret;
}

static void free_leaf_list(struct ulist *blocks)
{
    struct ulist_node *node = NULL;
    struct extent_inode_elem *eie;
    struct extent_inode_elem *eie_next;
    struct ulist_iterator uiter;

    ULIST_ITER_INIT(&uiter);
    while ((node = ulist_next(blocks, &uiter))) {
        if (!node->aux)
            continue;
        eie = (struct extent_inode_elem *)(uintptr_t)node->aux;
        for (; eie; eie = eie_next) {
            eie_next = eie->next;
            kfree(eie);
        }
        node->aux = 0;
    }

    ulist_free(blocks);
}

/*
 * Finds all leafs with a reference to the specified combination of bytenr and
 * offset. key_list_head will point to a list of corresponding keys (caller must
 * free each list element). The leafs will be stored in the leafs ulist, which
 * must be freed with ulist_free.
 *
 * returns 0 on success, <0 on error
 */
static int btrfs_find_all_leafs(struct btrfs_trans_handle *trans,
                struct btrfs_fs_info *fs_info, u64 bytenr,
                u64 time_seq, struct ulist **leafs,
                const u64 *extent_item_pos)
{
    struct ulist *tmp;
    int ret;

    tmp = ulist_alloc(GFP_NOFS);
    if (!tmp)
        return -ENOMEM;
    *leafs = ulist_alloc(GFP_NOFS);
    if (!*leafs) {
        ulist_free(tmp);
        return -ENOMEM;
    }

    ret = find_parent_nodes(trans, fs_info, bytenr,
                time_seq, *leafs, tmp, extent_item_pos);
    ulist_free(tmp);

    if (ret < 0 && ret != -ENOENT) {
        free_leaf_list(*leafs);
        return ret;
    }

    return 0;
}

/*
 * walk all backrefs for a given extent to find all roots that reference this
 * extent. Walking a backref means finding all extents that reference this
 * extent and in turn walk the backrefs of those, too. Naturally this is a
 * recursive process, but here it is implemented in an iterative fashion: We
 * find all referencing extents for the extent in question and put them on a
 * list. In turn, we find all referencing extents for those, further appending
 * to the list. The way we iterate the list allows adding more elements after
 * the current while iterating. The process stops when we reach the end of the
 * list. Found roots are added to the roots list.
 *
 * returns 0 on success, < 0 on error.
 */
int btrfs_find_all_roots(struct btrfs_trans_handle *trans,
                struct btrfs_fs_info *fs_info, u64 bytenr,
                u64 time_seq, struct ulist **roots)
{
    struct ulist *tmp;
    struct ulist_node *node = NULL;
    struct ulist_iterator uiter;
    int ret;

    tmp = ulist_alloc(GFP_NOFS);
    if (!tmp)
        return -ENOMEM;
    *roots = ulist_alloc(GFP_NOFS);
    if (!*roots) {
        ulist_free(tmp);
        return -ENOMEM;
    }

    ULIST_ITER_INIT(&uiter);
    while (1) {
        ret = find_parent_nodes(trans, fs_info, bytenr,
                    time_seq, tmp, *roots, NULL);
        if (ret < 0 && ret != -ENOENT) {
            ulist_free(tmp);
            ulist_free(*roots);
            return ret;
        }
        node = ulist_next(tmp, &uiter);
        if (!node)
            break;
        bytenr = node->val;
    }

    ulist_free(tmp);
    return 0;
}


static int __inode_info(u64 inum, u64 ioff, u8 key_type,
            struct btrfs_root *fs_root, struct btrfs_path *path,
            struct btrfs_key *found_key)
{
    int ret;
    struct btrfs_key key;
    struct extent_buffer *eb;

    key.type = key_type;
    key.objectid = inum;
    key.offset = ioff;

    ret = btrfs_search_slot(NULL, fs_root, &key, path, 0, 0);
    if (ret < 0)
        return ret;

    eb = path->nodes[0];
    if (ret && path->slots[0] >= btrfs_header_nritems(eb)) {
        ret = btrfs_next_leaf(fs_root, path);
        if (ret)
            return ret;
        eb = path->nodes[0];
    }

    btrfs_item_key_to_cpu(eb, found_key, path->slots[0]);
    if (found_key->type != key.type || found_key->objectid != key.objectid)
        return 1;

    return 0;
}

/*
 * this makes the path point to (inum INODE_ITEM ioff)
 */
int inode_item_info(u64 inum, u64 ioff, struct btrfs_root *fs_root,
            struct btrfs_path *path)
{
    struct btrfs_key key;
    return __inode_info(inum, ioff, BTRFS_INODE_ITEM_KEY, fs_root, path,
                &key);
}

static int inode_ref_info(u64 inum, u64 ioff, struct btrfs_root *fs_root,
                struct btrfs_path *path,
                struct btrfs_key *found_key)
{
    return __inode_info(inum, ioff, BTRFS_INODE_REF_KEY, fs_root, path,
                found_key);
}

int btrfs_find_one_extref(struct btrfs_root *root, u64 inode_objectid,
              u64 start_off, struct btrfs_path *path,
              struct btrfs_inode_extref **ret_extref,
              u64 *found_off)
{
    int ret, slot;
    struct btrfs_key key;
    struct btrfs_key found_key;
    struct btrfs_inode_extref *extref;
    struct extent_buffer *leaf;
    unsigned long ptr;

    key.objectid = inode_objectid;
    btrfs_set_key_type(&key, BTRFS_INODE_EXTREF_KEY);
    key.offset = start_off;

    ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
    if (ret < 0)
        return ret;

    while (1) {
        leaf = path->nodes[0];
        slot = path->slots[0];
        if (slot >= btrfs_header_nritems(leaf)) {
            /*
             * If the item at offset is not found,
             * btrfs_search_slot will point us to the slot
             * where it should be inserted. In our case
             * that will be the slot directly before the
             * next INODE_REF_KEY_V2 item. In the case
             * that we're pointing to the last slot in a
             * leaf, we must move one leaf over.
             */
            ret = btrfs_next_leaf(root, path);
            if (ret) {
                if (ret >= 1)
                    ret = -ENOENT;
                break;
            }
            continue;
        }

        btrfs_item_key_to_cpu(leaf, &found_key, slot);

        /*
         * Check that we're still looking at an extended ref key for
         * this particular objectid. If we have different
         * objectid or type then there are no more to be found
         * in the tree and we can exit.
         */
        ret = -ENOENT;
        if (found_key.objectid != inode_objectid)
            break;
        if (btrfs_key_type(&found_key) != BTRFS_INODE_EXTREF_KEY)
            break;

        ret = 0;
        ptr = btrfs_item_ptr_offset(leaf, path->slots[0]);
        extref = (struct btrfs_inode_extref *)ptr;
        *ret_extref = extref;
        if (found_off)
            *found_off = found_key.offset;
        break;
    }

    return ret;
}

char *btrfs_ref_to_path(struct btrfs_root *fs_root, struct btrfs_path *path,
            u32 name_len, unsigned long name_off,
            struct extent_buffer *eb_in, u64 parent,
            char *dest, u32 size)
{
    int slot;
    u64 next_inum;
    int ret;
    s64 bytes_left = ((s64)size) - 1;
    struct extent_buffer *eb = eb_in;
    struct btrfs_key found_key;
    int leave_spinning = path->leave_spinning;
    struct btrfs_inode_ref *iref;

    if (bytes_left >= 0)
        dest[bytes_left] = '\0';

    path->leave_spinning = 1;
    while (1) {
        bytes_left -= name_len;
        if (bytes_left >= 0)
            read_extent_buffer(eb, dest + bytes_left,
                       name_off, name_len);
        if (eb != eb_in) {
            btrfs_tree_read_unlock_blocking(eb);
            free_extent_buffer(eb);
        }
        ret = inode_ref_info(parent, 0, fs_root, path, &found_key);
        if (ret > 0)
            ret = -ENOENT;
        if (ret)
            break;

        next_inum = found_key.offset;

        /* regular exit ahead */
        if (parent == next_inum)
            break;

        slot = path->slots[0];
        eb = path->nodes[0];
        /* make sure we can use eb after releasing the path */
        if (eb != eb_in) {
            atomic_inc(&eb->refs);
            btrfs_tree_read_lock(eb);
            btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
        }
        btrfs_release_path(path);
        iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);

        name_len = btrfs_inode_ref_name_len(eb, iref);
        name_off = (unsigned long)(iref + 1);

        parent = next_inum;
        --bytes_left;
        if (bytes_left >= 0)
            dest[bytes_left] = '/';
    }

    btrfs_release_path(path);
    path->leave_spinning = leave_spinning;

    if (ret)
        return ERR_PTR(ret);

    return dest + bytes_left;
}

/*
 * this iterates to turn a btrfs_inode_ref into a full filesystem path. elements
 * of the path are separated by '/' and the path is guaranteed to be
 * 0-terminated. the path is only given within the current file system.
 * Therefore, it never starts with a '/'. the caller is responsible to provide
 * "size" bytes in "dest". the dest buffer will be filled backwards. finally,
 * the start point of the resulting string is returned. this pointer is within
 * dest, normally.
 * in case the path buffer would overflow, the pointer is decremented further
 * as if output was written to the buffer, though no more output is actually
 * generated. that way, the caller can determine how much space would be
 * required for the path to fit into the buffer. in that case, the returned
 * value will be smaller than dest. callers must check this!
 */
char *btrfs_iref_to_path(struct btrfs_root *fs_root,
             struct btrfs_path *path,
             struct btrfs_inode_ref *iref,
             struct extent_buffer *eb_in, u64 parent,
             char *dest, u32 size)
{
    return btrfs_ref_to_path(fs_root, path,
                 btrfs_inode_ref_name_len(eb_in, iref),
                 (unsigned long)(iref + 1),
                 eb_in, parent, dest, size);
}

/*
 * this makes the path point to (logical EXTENT_ITEM *)
 * returns BTRFS_EXTENT_FLAG_DATA for data, BTRFS_EXTENT_FLAG_TREE_BLOCK for
 * tree blocks and <0 on error.
 */
int extent_from_logical(struct btrfs_fs_info *fs_info, u64 logical,
            struct btrfs_path *path, struct btrfs_key *found_key,
            u64 *flags_ret)
{
    int ret;
    u64 flags;
    u32 item_size;
    struct extent_buffer *eb;
    struct btrfs_extent_item *ei;
    struct btrfs_key key;

    key.type = BTRFS_EXTENT_ITEM_KEY;
    key.objectid = logical;
    key.offset = (u64)-1;

    ret = btrfs_search_slot(NULL, fs_info->extent_root, &key, path, 0, 0);
    if (ret < 0)
        return ret;
    ret = btrfs_previous_item(fs_info->extent_root, path,
                    0, BTRFS_EXTENT_ITEM_KEY);
    if (ret < 0)
        return ret;

    btrfs_item_key_to_cpu(path->nodes[0], found_key, path->slots[0]);
    if (found_key->type != BTRFS_EXTENT_ITEM_KEY ||
        found_key->objectid > logical ||
        found_key->objectid + found_key->offset <= logical) {
        pr_debug("logical %llu is not within any extent\n",
             (unsigned long long)logical);
        return -ENOENT;
    }

    eb = path->nodes[0];
    item_size = btrfs_item_size_nr(eb, path->slots[0]);
    BUG_ON(item_size < sizeof(*ei));

    ei = btrfs_item_ptr(eb, path->slots[0], struct btrfs_extent_item);
    flags = btrfs_extent_flags(eb, ei);

    pr_debug("logical %llu is at position %llu within the extent (%llu "
         "EXTENT_ITEM %llu) flags %#llx size %u\n",
         (unsigned long long)logical,
         (unsigned long long)(logical - found_key->objectid),
         (unsigned long long)found_key->objectid,
         (unsigned long long)found_key->offset,
         (unsigned long long)flags, item_size);

    WARN_ON(!flags_ret);
    if (flags_ret) {
        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
            *flags_ret = BTRFS_EXTENT_FLAG_TREE_BLOCK;
        else if (flags & BTRFS_EXTENT_FLAG_DATA)
            *flags_ret = BTRFS_EXTENT_FLAG_DATA;
        else
            BUG_ON(1);
        return 0;
    }

    return -EIO;
}

/*
 * helper function to iterate extent inline refs. ptr must point to a 0 value
 * for the first call and may be modified. it is used to track state.
 * if more refs exist, 0 is returned and the next call to
 * __get_extent_inline_ref must pass the modified ptr parameter to get the
 * next ref. after the last ref was processed, 1 is returned.
 * returns <0 on error
 */
static int __get_extent_inline_ref(unsigned long *ptr, struct extent_buffer *eb,
                struct btrfs_extent_item *ei, u32 item_size,
                struct btrfs_extent_inline_ref **out_eiref,
                int *out_type)
{
    unsigned long end;
    u64 flags;
    struct btrfs_tree_block_info *info;

    if (!*ptr) {
        /* first call */
        flags = btrfs_extent_flags(eb, ei);
        if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK) {
            info = (struct btrfs_tree_block_info *)(ei + 1);
            *out_eiref =
                (struct btrfs_extent_inline_ref *)(info + 1);
        } else {
            *out_eiref = (struct btrfs_extent_inline_ref *)(ei + 1);
        }
        *ptr = (unsigned long)*out_eiref;
        if ((void *)*ptr >= (void *)ei + item_size)
            return -ENOENT;
    }

    end = (unsigned long)ei + item_size;
    *out_eiref = (struct btrfs_extent_inline_ref *)*ptr;
    *out_type = btrfs_extent_inline_ref_type(eb, *out_eiref);

    *ptr += btrfs_extent_inline_ref_size(*out_type);
    WARN_ON(*ptr > end);
    if (*ptr == end)
        return 1; /* last */

    return 0;
}

/*
 * reads the tree block backref for an extent. tree level and root are returned
 * through out_level and out_root. ptr must point to a 0 value for the first
 * call and may be modified (see __get_extent_inline_ref comment).
 * returns 0 if data was provided, 1 if there was no more data to provide or
 * <0 on error.
 */
int tree_backref_for_extent(unsigned long *ptr, struct extent_buffer *eb,
                struct btrfs_extent_item *ei, u32 item_size,
                u64 *out_root, u8 *out_level)
{
    int ret;
    int type;
    struct btrfs_tree_block_info *info;
    struct btrfs_extent_inline_ref *eiref;

    if (*ptr == (unsigned long)-1)
        return 1;

    while (1) {
        ret = __get_extent_inline_ref(ptr, eb, ei, item_size,
                        &eiref, &type);
        if (ret < 0)
            return ret;

        if (type == BTRFS_TREE_BLOCK_REF_KEY ||
            type == BTRFS_SHARED_BLOCK_REF_KEY)
            break;

        if (ret == 1)
            return 1;
    }

    /* we can treat both ref types equally here */
    info = (struct btrfs_tree_block_info *)(ei + 1);
    *out_root = btrfs_extent_inline_ref_offset(eb, eiref);
    *out_level = btrfs_tree_block_level(eb, info);

    if (ret == 1)
        *ptr = (unsigned long)-1;

    return 0;
}

static int iterate_leaf_refs(struct extent_inode_elem *inode_list,
                u64 root, u64 extent_item_objectid,
                iterate_extent_inodes_t *iterate, void *ctx)
{
    struct extent_inode_elem *eie;
    int ret = 0;

    for (eie = inode_list; eie; eie = eie->next) {
        pr_debug("ref for %llu resolved, key (%llu EXTEND_DATA %llu), "
             "root %llu\n", extent_item_objectid,
             eie->inum, eie->offset, root);
        ret = iterate(eie->inum, eie->offset, root, ctx);
        if (ret) {
            pr_debug("stopping iteration for %llu due to ret=%d\n",
                 extent_item_objectid, ret);
            break;
        }
    }

    return ret;
}

/*
 * calls iterate() for every inode that references the extent identified by
 * the given parameters.
 * when the iterator function returns a non-zero value, iteration stops.
 */
int iterate_extent_inodes(struct btrfs_fs_info *fs_info,
                u64 extent_item_objectid, u64 extent_item_pos,
                int search_commit_root,
                iterate_extent_inodes_t *iterate, void *ctx)
{
    int ret;
    struct list_head data_refs = LIST_HEAD_INIT(data_refs);
    struct list_head shared_refs = LIST_HEAD_INIT(shared_refs);
    struct btrfs_trans_handle *trans;
    struct ulist *refs = NULL;
    struct ulist *roots = NULL;
    struct ulist_node *ref_node = NULL;
    struct ulist_node *root_node = NULL;
    struct seq_list tree_mod_seq_elem = {};
    struct ulist_iterator ref_uiter;
    struct ulist_iterator root_uiter;

    pr_debug("resolving all inodes for extent %llu\n",
            extent_item_objectid);

    if (search_commit_root) {
        trans = BTRFS_BACKREF_SEARCH_COMMIT_ROOT;
    } else {
        trans = btrfs_join_transaction(fs_info->extent_root);
        if (IS_ERR(trans))
            return PTR_ERR(trans);
        btrfs_get_tree_mod_seq(fs_info, &tree_mod_seq_elem);
    }

    ret = btrfs_find_all_leafs(trans, fs_info, extent_item_objectid,
                   tree_mod_seq_elem.seq, &refs,
                   &extent_item_pos);
    if (ret)
        goto out;

    ULIST_ITER_INIT(&ref_uiter);
    while (!ret && (ref_node = ulist_next(refs, &ref_uiter))) {
        ret = btrfs_find_all_roots(trans, fs_info, ref_node->val,
                       tree_mod_seq_elem.seq, &roots);
        if (ret)
            break;
        ULIST_ITER_INIT(&root_uiter);
        while (!ret && (root_node = ulist_next(roots, &root_uiter))) {
            pr_debug("root %llu references leaf %llu, data list "
                 "%#llx\n", root_node->val, ref_node->val,
                 (long long)ref_node->aux);
            ret = iterate_leaf_refs((struct extent_inode_elem *)
                        (uintptr_t)ref_node->aux,
                        root_node->val,
                        extent_item_objectid,
                        iterate, ctx);
        }
        ulist_free(roots);
        roots = NULL;
    }

    free_leaf_list(refs);
    ulist_free(roots);
out:
    if (!search_commit_root) {
        btrfs_put_tree_mod_seq(fs_info, &tree_mod_seq_elem);
        btrfs_end_transaction(trans, fs_info->extent_root);
    }

    return ret;
}

int iterate_inodes_from_logical(u64 logical, struct btrfs_fs_info *fs_info,
                struct btrfs_path *path,
                iterate_extent_inodes_t *iterate, void *ctx)
{
    int ret;
    u64 extent_item_pos;
    u64 flags = 0;
    struct btrfs_key found_key;
    int search_commit_root = path->search_commit_root;

    ret = extent_from_logical(fs_info, logical, path, &found_key, &flags);
    btrfs_release_path(path);
    if (ret < 0)
        return ret;
    if (flags & BTRFS_EXTENT_FLAG_TREE_BLOCK)
        return -EINVAL;

    extent_item_pos = logical - found_key.objectid;
    ret = iterate_extent_inodes(fs_info, found_key.objectid,
                    extent_item_pos, search_commit_root,
                    iterate, ctx);

    return ret;
}

typedef int (iterate_irefs_t)(u64 parent, u32 name_len, unsigned long name_off,
                  struct extent_buffer *eb, void *ctx);

static int iterate_inode_refs(u64 inum, struct btrfs_root *fs_root,
                  struct btrfs_path *path,
                  iterate_irefs_t *iterate, void *ctx)
{
    int ret = 0;
    int slot;
    u32 cur;
    u32 len;
    u32 name_len;
    u64 parent = 0;
    int found = 0;
    struct extent_buffer *eb;
    struct btrfs_item *item;
    struct btrfs_inode_ref *iref;
    struct btrfs_key found_key;

    while (!ret) {
        path->leave_spinning = 1;
        ret = inode_ref_info(inum, parent ? parent+1 : 0, fs_root, path,
                     &found_key);
        if (ret < 0)
            break;
        if (ret) {
            ret = found ? 0 : -ENOENT;
            break;
        }
        ++found;

        parent = found_key.offset;
        slot = path->slots[0];
        eb = path->nodes[0];
        /* make sure we can use eb after releasing the path */
        atomic_inc(&eb->refs);
        btrfs_tree_read_lock(eb);
        btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
        btrfs_release_path(path);

        item = btrfs_item_nr(eb, slot);
        iref = btrfs_item_ptr(eb, slot, struct btrfs_inode_ref);

        for (cur = 0; cur < btrfs_item_size(eb, item); cur += len) {
            name_len = btrfs_inode_ref_name_len(eb, iref);
            /* path must be released before calling iterate()! */
            pr_debug("following ref at offset %u for inode %llu in "
                 "tree %llu\n", cur,
                 (unsigned long long)found_key.objectid,
                 (unsigned long long)fs_root->objectid);
            ret = iterate(parent, name_len,
                      (unsigned long)(iref + 1), eb, ctx);
            if (ret)
                break;
            len = sizeof(*iref) + name_len;
            iref = (struct btrfs_inode_ref *)((char *)iref + len);
        }
        btrfs_tree_read_unlock_blocking(eb);
        free_extent_buffer(eb);
    }

    btrfs_release_path(path);

    return ret;
}

static int iterate_inode_extrefs(u64 inum, struct btrfs_root *fs_root,
                 struct btrfs_path *path,
                 iterate_irefs_t *iterate, void *ctx)
{
    int ret;
    int slot;
    u64 offset = 0;
    u64 parent;
    int found = 0;
    struct extent_buffer *eb;
    struct btrfs_inode_extref *extref;
    struct extent_buffer *leaf;
    u32 item_size;
    u32 cur_offset;
    unsigned long ptr;

    while (1) {
        ret = btrfs_find_one_extref(fs_root, inum, offset, path, &extref,
                        &offset);
        if (ret < 0)
            break;
        if (ret) {
            ret = found ? 0 : -ENOENT;
            break;
        }
        ++found;

        slot = path->slots[0];
        eb = path->nodes[0];
        /* make sure we can use eb after releasing the path */
        atomic_inc(&eb->refs);

        btrfs_tree_read_lock(eb);
        btrfs_set_lock_blocking_rw(eb, BTRFS_READ_LOCK);
        btrfs_release_path(path);

        leaf = path->nodes[0];
        item_size = btrfs_item_size_nr(leaf, path->slots[0]);
        ptr = btrfs_item_ptr_offset(leaf, path->slots[0]);
        cur_offset = 0;

        while (cur_offset < item_size) {
            u32 name_len;

            extref = (struct btrfs_inode_extref *)(ptr + cur_offset);
            parent = btrfs_inode_extref_parent(eb, extref);
            name_len = btrfs_inode_extref_name_len(eb, extref);
            ret = iterate(parent, name_len,
                      (unsigned long)&extref->name, eb, ctx);
            if (ret)
                break;

            cur_offset += btrfs_inode_extref_name_len(leaf, extref);
            cur_offset += sizeof(*extref);
        }
        btrfs_tree_read_unlock_blocking(eb);
        free_extent_buffer(eb);

        offset++;
    }

    btrfs_release_path(path);

    return ret;
}

static int iterate_irefs(u64 inum, struct btrfs_root *fs_root,
             struct btrfs_path *path, iterate_irefs_t *iterate,
             void *ctx)
{
    int ret;
    int found_refs = 0;

    ret = iterate_inode_refs(inum, fs_root, path, iterate, ctx);
    if (!ret)
        ++found_refs;
    else if (ret != -ENOENT)
        return ret;

    ret = iterate_inode_extrefs(inum, fs_root, path, iterate, ctx);
    if (ret == -ENOENT && found_refs)
        return 0;

    return ret;
}

/*
 * returns 0 if the path could be dumped (probably truncated)
 * returns <0 in case of an error
 */
static int inode_to_path(u64 inum, u32 name_len, unsigned long name_off,
             struct extent_buffer *eb, void *ctx)
{
    struct inode_fs_paths *ipath = ctx;
    char *fspath;
    char *fspath_min;
    int i = ipath->fspath->elem_cnt;
    const int s_ptr = sizeof(char *);
    u32 bytes_left;

    bytes_left = ipath->fspath->bytes_left > s_ptr ?
                    ipath->fspath->bytes_left - s_ptr : 0;

    fspath_min = (char *)ipath->fspath->val + (i + 1) * s_ptr;
    fspath = btrfs_ref_to_path(ipath->fs_root, ipath->btrfs_path, name_len,
                   name_off, eb, inum, fspath_min, bytes_left);
    if (IS_ERR(fspath))
        return PTR_ERR(fspath);

    if (fspath > fspath_min) {
        ipath->fspath->val[i] = (u64)(unsigned long)fspath;
        ++ipath->fspath->elem_cnt;
        ipath->fspath->bytes_left = fspath - fspath_min;
    } else {
        ++ipath->fspath->elem_missed;
        ipath->fspath->bytes_missing += fspath_min - fspath;
        ipath->fspath->bytes_left = 0;
    }

    return 0;
}

/*
 * this dumps all file system paths to the inode into the ipath struct, provided
 * is has been created large enough. each path is zero-terminated and accessed
 * from ipath->fspath->val[i].
 * when it returns, there are ipath->fspath->elem_cnt number of paths available
 * in ipath->fspath->val[]. when the allocated space wasn't sufficient, the
 * number of missed paths in recored in ipath->fspath->elem_missed, otherwise,
 * it's zero. ipath->fspath->bytes_missing holds the number of bytes that would
 * have been needed to return all paths.
 */
int paths_from_inode(u64 inum, struct inode_fs_paths *ipath)
{
    return iterate_irefs(inum, ipath->fs_root, ipath->btrfs_path,
                 inode_to_path, ipath);
}

struct btrfs_data_container *init_data_container(u32 total_bytes)
{
    struct btrfs_data_container *data;
    size_t alloc_bytes;

    alloc_bytes = max_t(size_t, total_bytes, sizeof(*data));
    data = vmalloc(alloc_bytes);
    if (!data)
        return ERR_PTR(-ENOMEM);

    if (total_bytes >= sizeof(*data)) {
        data->bytes_left = total_bytes - sizeof(*data);
        data->bytes_missing = 0;
    } else {
        data->bytes_missing = sizeof(*data) - total_bytes;
        data->bytes_left = 0;
    }

    data->elem_cnt = 0;
    data->elem_missed = 0;

    return data;
}

/*
 * allocates space to return multiple file system paths for an inode.
 * total_bytes to allocate are passed, note that space usable for actual path
 * information will be total_bytes - sizeof(struct inode_fs_paths).
 * the returned pointer must be freed with free_ipath() in the end.
 */
struct inode_fs_paths *init_ipath(s32 total_bytes, struct btrfs_root *fs_root,
                    struct btrfs_path *path)
{
    struct inode_fs_paths *ifp;
    struct btrfs_data_container *fspath;

    fspath = init_data_container(total_bytes);
    if (IS_ERR(fspath))
        return (void *)fspath;

    ifp = kmalloc(sizeof(*ifp), GFP_NOFS);
    if (!ifp) {
        kfree(fspath);
        return ERR_PTR(-ENOMEM);
    }

    ifp->btrfs_path = path;
    ifp->fspath = fspath;
    ifp->fs_root = fs_root;

    return ifp;
}

void free_ipath(struct inode_fs_paths *ipath)
{
    if (!ipath)
        return;
    vfree(ipath->fspath);
    kfree(ipath);
}