delayed-ref.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/slab.h>
#include <linux/sort.h>
#include "ctree.h"
#include "delayed-ref.h"
#include "transaction.h"

/*
 * delayed back reference update tracking.  For subvolume trees
 * we queue up extent allocations and backref maintenance for
 * delayed processing.   This avoids deep call chains where we
 * add extents in the middle of btrfs_search_slot, and it allows
 * us to buffer up frequently modified backrefs in an rb tree instead
 * of hammering updates on the extent allocation tree.
 */

/*
 * compare two delayed tree backrefs with same bytenr and type
 */
static int comp_tree_refs(struct btrfs_delayed_tree_ref *ref2,
              struct btrfs_delayed_tree_ref *ref1)
{
    if (ref1->root < ref2->root)
        return -1;
    if (ref1->root > ref2->root)
        return 1;
    if (ref1->parent < ref2->parent)
        return -1;
    if (ref1->parent > ref2->parent)
        return 1;
    return 0;
}

/*
 * compare two delayed data backrefs with same bytenr and type
 */
static int comp_data_refs(struct btrfs_delayed_data_ref *ref2,
              struct btrfs_delayed_data_ref *ref1)
{
    if (ref1->node.type == BTRFS_EXTENT_DATA_REF_KEY) {
        if (ref1->root < ref2->root)
            return -1;
        if (ref1->root > ref2->root)
            return 1;
        if (ref1->objectid < ref2->objectid)
            return -1;
        if (ref1->objectid > ref2->objectid)
            return 1;
        if (ref1->offset < ref2->offset)
            return -1;
        if (ref1->offset > ref2->offset)
            return 1;
    } else {
        if (ref1->parent < ref2->parent)
            return -1;
        if (ref1->parent > ref2->parent)
            return 1;
    }
    return 0;
}

/*
 * entries in the rb tree are ordered by the byte number of the extent,
 * type of the delayed backrefs and content of delayed backrefs.
 */
static int comp_entry(struct btrfs_delayed_ref_node *ref2,
              struct btrfs_delayed_ref_node *ref1,
              bool compare_seq)
{
    if (ref1->bytenr < ref2->bytenr)
        return -1;
    if (ref1->bytenr > ref2->bytenr)
        return 1;
    if (ref1->is_head && ref2->is_head)
        return 0;
    if (ref2->is_head)
        return -1;
    if (ref1->is_head)
        return 1;
    if (ref1->type < ref2->type)
        return -1;
    if (ref1->type > ref2->type)
        return 1;
    /* merging of sequenced refs is not allowed */
    if (compare_seq) {
        if (ref1->seq < ref2->seq)
            return -1;
        if (ref1->seq > ref2->seq)
            return 1;
    }
    if (ref1->type == BTRFS_TREE_BLOCK_REF_KEY ||
        ref1->type == BTRFS_SHARED_BLOCK_REF_KEY) {
        return comp_tree_refs(btrfs_delayed_node_to_tree_ref(ref2),
                      btrfs_delayed_node_to_tree_ref(ref1));
    } else if (ref1->type == BTRFS_EXTENT_DATA_REF_KEY ||
           ref1->type == BTRFS_SHARED_DATA_REF_KEY) {
        return comp_data_refs(btrfs_delayed_node_to_data_ref(ref2),
                      btrfs_delayed_node_to_data_ref(ref1));
    }
    BUG();
    return 0;
}

/*
 * insert a new ref into the rbtree.  This returns any existing refs
 * for the same (bytenr,parent) tuple, or NULL if the new node was properly
 * inserted.
 */
static struct btrfs_delayed_ref_node *tree_insert(struct rb_root *root,
                          struct rb_node *node)
{
    struct rb_node **p = &root->rb_node;
    struct rb_node *parent_node = NULL;
    struct btrfs_delayed_ref_node *entry;
    struct btrfs_delayed_ref_node *ins;
    int cmp;

    ins = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
    while (*p) {
        parent_node = *p;
        entry = rb_entry(parent_node, struct btrfs_delayed_ref_node,
                 rb_node);

        cmp = comp_entry(entry, ins, 1);
        if (cmp < 0)
            p = &(*p)->rb_left;
        else if (cmp > 0)
            p = &(*p)->rb_right;
        else
            return entry;
    }

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

/*
 * find an head entry based on bytenr. This returns the delayed ref
 * head if it was able to find one, or NULL if nothing was in that spot.
 * If return_bigger is given, the next bigger entry is returned if no exact
 * match is found.
 */
static struct btrfs_delayed_ref_node *find_ref_head(struct rb_root *root,
                  u64 bytenr,
                  struct btrfs_delayed_ref_node **last,
                  int return_bigger)
{
    struct rb_node *n;
    struct btrfs_delayed_ref_node *entry;
    int cmp = 0;

again:
    n = root->rb_node;
    entry = NULL;
    while (n) {
        entry = rb_entry(n, struct btrfs_delayed_ref_node, rb_node);
        WARN_ON(!entry->in_tree);
        if (last)
            *last = entry;

        if (bytenr < entry->bytenr)
            cmp = -1;
        else if (bytenr > entry->bytenr)
            cmp = 1;
        else if (!btrfs_delayed_ref_is_head(entry))
            cmp = 1;
        else
            cmp = 0;

        if (cmp < 0)
            n = n->rb_left;
        else if (cmp > 0)
            n = n->rb_right;
        else
            return entry;
    }
    if (entry && return_bigger) {
        if (cmp > 0) {
            n = rb_next(&entry->rb_node);
            if (!n)
                n = rb_first(root);
            entry = rb_entry(n, struct btrfs_delayed_ref_node,
                     rb_node);
            bytenr = entry->bytenr;
            return_bigger = 0;
            goto again;
        }
        return entry;
    }
    return NULL;
}

int btrfs_delayed_ref_lock(struct btrfs_trans_handle *trans,
               struct btrfs_delayed_ref_head *head)
{
    struct btrfs_delayed_ref_root *delayed_refs;

    delayed_refs = &trans->transaction->delayed_refs;
    assert_spin_locked(&delayed_refs->lock);
    if (mutex_trylock(&head->mutex))
        return 0;

    atomic_inc(&head->node.refs);
    spin_unlock(&delayed_refs->lock);

    mutex_lock(&head->mutex);
    spin_lock(&delayed_refs->lock);
    if (!head->node.in_tree) {
        mutex_unlock(&head->mutex);
        btrfs_put_delayed_ref(&head->node);
        return -EAGAIN;
    }
    btrfs_put_delayed_ref(&head->node);
    return 0;
}

static void inline drop_delayed_ref(struct btrfs_trans_handle *trans,
                    struct btrfs_delayed_ref_root *delayed_refs,
                    struct btrfs_delayed_ref_node *ref)
{
    rb_erase(&ref->rb_node, &delayed_refs->root);
    ref->in_tree = 0;
    btrfs_put_delayed_ref(ref);
    delayed_refs->num_entries--;
    if (trans->delayed_ref_updates)
        trans->delayed_ref_updates--;
}

static int merge_ref(struct btrfs_trans_handle *trans,
             struct btrfs_delayed_ref_root *delayed_refs,
             struct btrfs_delayed_ref_node *ref, u64 seq)
{
    struct rb_node *node;
    int merged = 0;
    int mod = 0;
    int done = 0;

    node = rb_prev(&ref->rb_node);
    while (node) {
        struct btrfs_delayed_ref_node *next;

        next = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
        node = rb_prev(node);
        if (next->bytenr != ref->bytenr)
            break;
        if (seq && next->seq >= seq)
            break;
        if (comp_entry(ref, next, 0))
            continue;

        if (ref->action == next->action) {
            mod = next->ref_mod;
        } else {
            if (ref->ref_mod < next->ref_mod) {
                struct btrfs_delayed_ref_node *tmp;

                tmp = ref;
                ref = next;
                next = tmp;
                done = 1;
            }
            mod = -next->ref_mod;
        }

        merged++;
        drop_delayed_ref(trans, delayed_refs, next);
        ref->ref_mod += mod;
        if (ref->ref_mod == 0) {
            drop_delayed_ref(trans, delayed_refs, ref);
            break;
        } else {
            /*
             * You can't have multiples of the same ref on a tree
             * block.
             */
            WARN_ON(ref->type == BTRFS_TREE_BLOCK_REF_KEY ||
                ref->type == BTRFS_SHARED_BLOCK_REF_KEY);
        }

        if (done)
            break;
        node = rb_prev(&ref->rb_node);
    }

    return merged;
}

void btrfs_merge_delayed_refs(struct btrfs_trans_handle *trans,
                  struct btrfs_fs_info *fs_info,
                  struct btrfs_delayed_ref_root *delayed_refs,
                  struct btrfs_delayed_ref_head *head)
{
    struct rb_node *node;
    u64 seq = 0;

    spin_lock(&fs_info->tree_mod_seq_lock);
    if (!list_empty(&fs_info->tree_mod_seq_list)) {
        struct seq_list *elem;

        elem = list_first_entry(&fs_info->tree_mod_seq_list,
                    struct seq_list, list);
        seq = elem->seq;
    }
    spin_unlock(&fs_info->tree_mod_seq_lock);

    node = rb_prev(&head->node.rb_node);
    while (node) {
        struct btrfs_delayed_ref_node *ref;

        ref = rb_entry(node, struct btrfs_delayed_ref_node,
                   rb_node);
        if (ref->bytenr != head->node.bytenr)
            break;

        /* We can't merge refs that are outside of our seq count */
        if (seq && ref->seq >= seq)
            break;
        if (merge_ref(trans, delayed_refs, ref, seq))
            node = rb_prev(&head->node.rb_node);
        else
            node = rb_prev(node);
    }
}

int btrfs_check_delayed_seq(struct btrfs_fs_info *fs_info,
                struct btrfs_delayed_ref_root *delayed_refs,
                u64 seq)
{
    struct seq_list *elem;
    int ret = 0;

    spin_lock(&fs_info->tree_mod_seq_lock);
    if (!list_empty(&fs_info->tree_mod_seq_list)) {
        elem = list_first_entry(&fs_info->tree_mod_seq_list,
                    struct seq_list, list);
        if (seq >= elem->seq) {
            pr_debug("holding back delayed_ref %llu, lowest is "
                 "%llu (%p)\n", seq, elem->seq, delayed_refs);
            ret = 1;
        }
    }

    spin_unlock(&fs_info->tree_mod_seq_lock);
    return ret;
}

int btrfs_find_ref_cluster(struct btrfs_trans_handle *trans,
               struct list_head *cluster, u64 start)
{
    int count = 0;
    struct btrfs_delayed_ref_root *delayed_refs;
    struct rb_node *node;
    struct btrfs_delayed_ref_node *ref;
    struct btrfs_delayed_ref_head *head;

    delayed_refs = &trans->transaction->delayed_refs;
    if (start == 0) {
        node = rb_first(&delayed_refs->root);
    } else {
        ref = NULL;
        find_ref_head(&delayed_refs->root, start + 1, &ref, 1);
        if (ref) {
            node = &ref->rb_node;
        } else
            node = rb_first(&delayed_refs->root);
    }
again:
    while (node && count < 32) {
        ref = rb_entry(node, struct btrfs_delayed_ref_node, rb_node);
        if (btrfs_delayed_ref_is_head(ref)) {
            head = btrfs_delayed_node_to_head(ref);
            if (list_empty(&head->cluster)) {
                list_add_tail(&head->cluster, cluster);
                delayed_refs->run_delayed_start =
                    head->node.bytenr;
                count++;

                WARN_ON(delayed_refs->num_heads_ready == 0);
                delayed_refs->num_heads_ready--;
            } else if (count) {
                /* the goal of the clustering is to find extents
                 * that are likely to end up in the same extent
                 * leaf on disk.  So, we don't want them spread
                 * all over the tree.  Stop now if we've hit
                 * a head that was already in use
                 */
                break;
            }
        }
        node = rb_next(node);
    }
    if (count) {
        return 0;
    } else if (start) {
        /*
         * we've gone to the end of the rbtree without finding any
         * clusters.  start from the beginning and try again
         */
        start = 0;
        node = rb_first(&delayed_refs->root);
        goto again;
    }
    return 1;
}

/*
 * helper function to update an extent delayed ref in the
 * rbtree.  existing and update must both have the same
 * bytenr and parent
 *
 * This may free existing if the update cancels out whatever
 * operation it was doing.
 */
static noinline void
update_existing_ref(struct btrfs_trans_handle *trans,
            struct btrfs_delayed_ref_root *delayed_refs,
            struct btrfs_delayed_ref_node *existing,
            struct btrfs_delayed_ref_node *update)
{
    if (update->action != existing->action) {
        /*
         * this is effectively undoing either an add or a
         * drop.  We decrement the ref_mod, and if it goes
         * down to zero we just delete the entry without
         * every changing the extent allocation tree.
         */
        existing->ref_mod--;
        if (existing->ref_mod == 0)
            drop_delayed_ref(trans, delayed_refs, existing);
        else
            WARN_ON(existing->type == BTRFS_TREE_BLOCK_REF_KEY ||
                existing->type == BTRFS_SHARED_BLOCK_REF_KEY);
    } else {
        WARN_ON(existing->type == BTRFS_TREE_BLOCK_REF_KEY ||
            existing->type == BTRFS_SHARED_BLOCK_REF_KEY);
        /*
         * the action on the existing ref matches
         * the action on the ref we're trying to add.
         * Bump the ref_mod by one so the backref that
         * is eventually added/removed has the correct
         * reference count
         */
        existing->ref_mod += update->ref_mod;
    }
}

/*
 * helper function to update the accounting in the head ref
 * existing and update must have the same bytenr
 */
static noinline void
update_existing_head_ref(struct btrfs_delayed_ref_node *existing,
             struct btrfs_delayed_ref_node *update)
{
    struct btrfs_delayed_ref_head *existing_ref;
    struct btrfs_delayed_ref_head *ref;

    existing_ref = btrfs_delayed_node_to_head(existing);
    ref = btrfs_delayed_node_to_head(update);
    BUG_ON(existing_ref->is_data != ref->is_data);

    if (ref->must_insert_reserved) {
        /* if the extent was freed and then
         * reallocated before the delayed ref
         * entries were processed, we can end up
         * with an existing head ref without
         * the must_insert_reserved flag set.
         * Set it again here
         */
        existing_ref->must_insert_reserved = ref->must_insert_reserved;

        /*
         * update the num_bytes so we make sure the accounting
         * is done correctly
         */
        existing->num_bytes = update->num_bytes;

    }

    if (ref->extent_op) {
        if (!existing_ref->extent_op) {
            existing_ref->extent_op = ref->extent_op;
        } else {
            if (ref->extent_op->update_key) {
                memcpy(&existing_ref->extent_op->key,
                       &ref->extent_op->key,
                       sizeof(ref->extent_op->key));
                existing_ref->extent_op->update_key = 1;
            }
            if (ref->extent_op->update_flags) {
                existing_ref->extent_op->flags_to_set |=
                    ref->extent_op->flags_to_set;
                existing_ref->extent_op->update_flags = 1;
            }
            kfree(ref->extent_op);
        }
    }
    /*
     * update the reference mod on the head to reflect this new operation
     */
    existing->ref_mod += update->ref_mod;
}

/*
 * helper function to actually insert a head node into the rbtree.
 * this does all the dirty work in terms of maintaining the correct
 * overall modification count.
 */
static noinline void add_delayed_ref_head(struct btrfs_fs_info *fs_info,
                    struct btrfs_trans_handle *trans,
                    struct btrfs_delayed_ref_node *ref,
                    u64 bytenr, u64 num_bytes,
                    int action, int is_data)
{
    struct btrfs_delayed_ref_node *existing;
    struct btrfs_delayed_ref_head *head_ref = NULL;
    struct btrfs_delayed_ref_root *delayed_refs;
    int count_mod = 1;
    int must_insert_reserved = 0;

    /*
     * the head node stores the sum of all the mods, so dropping a ref
     * should drop the sum in the head node by one.
     */
    if (action == BTRFS_UPDATE_DELAYED_HEAD)
        count_mod = 0;
    else if (action == BTRFS_DROP_DELAYED_REF)
        count_mod = -1;

    /*
     * BTRFS_ADD_DELAYED_EXTENT means that we need to update
     * the reserved accounting when the extent is finally added, or
     * if a later modification deletes the delayed ref without ever
     * inserting the extent into the extent allocation tree.
     * ref->must_insert_reserved is the flag used to record
     * that accounting mods are required.
     *
     * Once we record must_insert_reserved, switch the action to
     * BTRFS_ADD_DELAYED_REF because other special casing is not required.
     */
    if (action == BTRFS_ADD_DELAYED_EXTENT)
        must_insert_reserved = 1;
    else
        must_insert_reserved = 0;

    delayed_refs = &trans->transaction->delayed_refs;

    /* first set the basic ref node struct up */
    atomic_set(&ref->refs, 1);
    ref->bytenr = bytenr;
    ref->num_bytes = num_bytes;
    ref->ref_mod = count_mod;
    ref->type  = 0;
    ref->action  = 0;
    ref->is_head = 1;
    ref->in_tree = 1;
    ref->seq = 0;

    head_ref = btrfs_delayed_node_to_head(ref);
    head_ref->must_insert_reserved = must_insert_reserved;
    head_ref->is_data = is_data;

    INIT_LIST_HEAD(&head_ref->cluster);
    mutex_init(&head_ref->mutex);

    trace_btrfs_delayed_ref_head(ref, head_ref, action);

    existing = tree_insert(&delayed_refs->root, &ref->rb_node);

    if (existing) {
        update_existing_head_ref(existing, ref);
        /*
         * we've updated the existing ref, free the newly
         * allocated ref
         */
        kfree(head_ref);
    } else {
        delayed_refs->num_heads++;
        delayed_refs->num_heads_ready++;
        delayed_refs->num_entries++;
        trans->delayed_ref_updates++;
    }
}

/*
 * helper to insert a delayed tree ref into the rbtree.
 */
static noinline void add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
                     struct btrfs_trans_handle *trans,
                     struct btrfs_delayed_ref_node *ref,
                     u64 bytenr, u64 num_bytes, u64 parent,
                     u64 ref_root, int level, int action,
                     int for_cow)
{
    struct btrfs_delayed_ref_node *existing;
    struct btrfs_delayed_tree_ref *full_ref;
    struct btrfs_delayed_ref_root *delayed_refs;
    u64 seq = 0;

    if (action == BTRFS_ADD_DELAYED_EXTENT)
        action = BTRFS_ADD_DELAYED_REF;

    delayed_refs = &trans->transaction->delayed_refs;

    /* first set the basic ref node struct up */
    atomic_set(&ref->refs, 1);
    ref->bytenr = bytenr;
    ref->num_bytes = num_bytes;
    ref->ref_mod = 1;
    ref->action = action;
    ref->is_head = 0;
    ref->in_tree = 1;

    if (need_ref_seq(for_cow, ref_root))
        seq = btrfs_get_tree_mod_seq(fs_info, &trans->delayed_ref_elem);
    ref->seq = seq;

    full_ref = btrfs_delayed_node_to_tree_ref(ref);
    full_ref->parent = parent;
    full_ref->root = ref_root;
    if (parent)
        ref->type = BTRFS_SHARED_BLOCK_REF_KEY;
    else
        ref->type = BTRFS_TREE_BLOCK_REF_KEY;
    full_ref->level = level;

    trace_btrfs_delayed_tree_ref(ref, full_ref, action);

    existing = tree_insert(&delayed_refs->root, &ref->rb_node);

    if (existing) {
        update_existing_ref(trans, delayed_refs, existing, ref);
        /*
         * we've updated the existing ref, free the newly
         * allocated ref
         */
        kfree(full_ref);
    } else {
        delayed_refs->num_entries++;
        trans->delayed_ref_updates++;
    }
}

/*
 * helper to insert a delayed data ref into the rbtree.
 */
static noinline void add_delayed_data_ref(struct btrfs_fs_info *fs_info,
                     struct btrfs_trans_handle *trans,
                     struct btrfs_delayed_ref_node *ref,
                     u64 bytenr, u64 num_bytes, u64 parent,
                     u64 ref_root, u64 owner, u64 offset,
                     int action, int for_cow)
{
    struct btrfs_delayed_ref_node *existing;
    struct btrfs_delayed_data_ref *full_ref;
    struct btrfs_delayed_ref_root *delayed_refs;
    u64 seq = 0;

    if (action == BTRFS_ADD_DELAYED_EXTENT)
        action = BTRFS_ADD_DELAYED_REF;

    delayed_refs = &trans->transaction->delayed_refs;

    /* first set the basic ref node struct up */
    atomic_set(&ref->refs, 1);
    ref->bytenr = bytenr;
    ref->num_bytes = num_bytes;
    ref->ref_mod = 1;
    ref->action = action;
    ref->is_head = 0;
    ref->in_tree = 1;

    if (need_ref_seq(for_cow, ref_root))
        seq = btrfs_get_tree_mod_seq(fs_info, &trans->delayed_ref_elem);
    ref->seq = seq;

    full_ref = btrfs_delayed_node_to_data_ref(ref);
    full_ref->parent = parent;
    full_ref->root = ref_root;
    if (parent)
        ref->type = BTRFS_SHARED_DATA_REF_KEY;
    else
        ref->type = BTRFS_EXTENT_DATA_REF_KEY;

    full_ref->objectid = owner;
    full_ref->offset = offset;

    trace_btrfs_delayed_data_ref(ref, full_ref, action);

    existing = tree_insert(&delayed_refs->root, &ref->rb_node);

    if (existing) {
        update_existing_ref(trans, delayed_refs, existing, ref);
        /*
         * we've updated the existing ref, free the newly
         * allocated ref
         */
        kfree(full_ref);
    } else {
        delayed_refs->num_entries++;
        trans->delayed_ref_updates++;
    }
}

/*
 * add a delayed tree ref.  This does all of the accounting required
 * to make sure the delayed ref is eventually processed before this
 * transaction commits.
 */
int btrfs_add_delayed_tree_ref(struct btrfs_fs_info *fs_info,
                   struct btrfs_trans_handle *trans,
                   u64 bytenr, u64 num_bytes, u64 parent,
                   u64 ref_root,  int level, int action,
                   struct btrfs_delayed_extent_op *extent_op,
                   int for_cow)
{
    struct btrfs_delayed_tree_ref *ref;
    struct btrfs_delayed_ref_head *head_ref;
    struct btrfs_delayed_ref_root *delayed_refs;

    BUG_ON(extent_op && extent_op->is_data);
    ref = kmalloc(sizeof(*ref), GFP_NOFS);
    if (!ref)
        return -ENOMEM;

    head_ref = kmalloc(sizeof(*head_ref), GFP_NOFS);
    if (!head_ref) {
        kfree(ref);
        return -ENOMEM;
    }

    head_ref->extent_op = extent_op;

    delayed_refs = &trans->transaction->delayed_refs;
    spin_lock(&delayed_refs->lock);

    /*
     * insert both the head node and the new ref without dropping
     * the spin lock
     */
    add_delayed_ref_head(fs_info, trans, &head_ref->node, bytenr,
                   num_bytes, action, 0);

    add_delayed_tree_ref(fs_info, trans, &ref->node, bytenr,
                   num_bytes, parent, ref_root, level, action,
                   for_cow);
    spin_unlock(&delayed_refs->lock);
    if (need_ref_seq(for_cow, ref_root))
        btrfs_qgroup_record_ref(trans, &ref->node, extent_op);

    return 0;
}

/*
 * add a delayed data ref. it's similar to btrfs_add_delayed_tree_ref.
 */
int btrfs_add_delayed_data_ref(struct btrfs_fs_info *fs_info,
                   struct btrfs_trans_handle *trans,
                   u64 bytenr, u64 num_bytes,
                   u64 parent, u64 ref_root,
                   u64 owner, u64 offset, int action,
                   struct btrfs_delayed_extent_op *extent_op,
                   int for_cow)
{
    struct btrfs_delayed_data_ref *ref;
    struct btrfs_delayed_ref_head *head_ref;
    struct btrfs_delayed_ref_root *delayed_refs;

    BUG_ON(extent_op && !extent_op->is_data);
    ref = kmalloc(sizeof(*ref), GFP_NOFS);
    if (!ref)
        return -ENOMEM;

    head_ref = kmalloc(sizeof(*head_ref), GFP_NOFS);
    if (!head_ref) {
        kfree(ref);
        return -ENOMEM;
    }

    head_ref->extent_op = extent_op;

    delayed_refs = &trans->transaction->delayed_refs;
    spin_lock(&delayed_refs->lock);

    /*
     * insert both the head node and the new ref without dropping
     * the spin lock
     */
    add_delayed_ref_head(fs_info, trans, &head_ref->node, bytenr,
                   num_bytes, action, 1);

    add_delayed_data_ref(fs_info, trans, &ref->node, bytenr,
                   num_bytes, parent, ref_root, owner, offset,
                   action, for_cow);
    spin_unlock(&delayed_refs->lock);
    if (need_ref_seq(for_cow, ref_root))
        btrfs_qgroup_record_ref(trans, &ref->node, extent_op);

    return 0;
}

int btrfs_add_delayed_extent_op(struct btrfs_fs_info *fs_info,
                struct btrfs_trans_handle *trans,
                u64 bytenr, u64 num_bytes,
                struct btrfs_delayed_extent_op *extent_op)
{
    struct btrfs_delayed_ref_head *head_ref;
    struct btrfs_delayed_ref_root *delayed_refs;

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

    head_ref->extent_op = extent_op;

    delayed_refs = &trans->transaction->delayed_refs;
    spin_lock(&delayed_refs->lock);

    add_delayed_ref_head(fs_info, trans, &head_ref->node, bytenr,
                   num_bytes, BTRFS_UPDATE_DELAYED_HEAD,
                   extent_op->is_data);

    spin_unlock(&delayed_refs->lock);
    return 0;
}

/*
 * this does a simple search for the head node for a given extent.
 * It must be called with the delayed ref spinlock held, and it returns
 * the head node if any where found, or NULL if not.
 */
struct btrfs_delayed_ref_head *
btrfs_find_delayed_ref_head(struct btrfs_trans_handle *trans, u64 bytenr)
{
    struct btrfs_delayed_ref_node *ref;
    struct btrfs_delayed_ref_root *delayed_refs;

    delayed_refs = &trans->transaction->delayed_refs;
    ref = find_ref_head(&delayed_refs->root, bytenr, NULL, 0);
    if (ref)
        return btrfs_delayed_node_to_head(ref);
    return NULL;
}