Btrfs: update backrefs while dropping snapshot

The new backref format has restriction on type of backref item.  If a tree
block isn't referenced by its owner tree, full backrefs must be used for the
pointers in it. When a tree block loses its owner tree's reference, backrefs
for the pointers in it should be updated to full backrefs. Current
btrfs_drop_snapshot misses the code that updates backrefs, so it's unsafe for
general use.

This patch adds backrefs update code to btrfs_drop_snapshot.  It isn't a
problem in the restricted form btrfs_drop_snapshot is used today, but for
general snapshot deletion this update is required.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 390 insertions, 174 deletions

diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index edc7d208c5ce..cd64cfc14f7f 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -990,15 +990,13 @@ static inline int extent_ref_type(u64 parent, u64 owner)
         return type;
 }
 
-static int find_next_key(struct btrfs_path *path, struct btrfs_key *key)
+static int find_next_key(struct btrfs_path *path, int level,
+                         struct btrfs_key *key)
 
 {
-        int level;
-        BUG_ON(!path->keep_locks);
-        for (level = 0; level < BTRFS_MAX_LEVEL; level++) {
+        for (; level < BTRFS_MAX_LEVEL; level++) {
                 if (!path->nodes[level])
                         break;
-                btrfs_assert_tree_locked(path->nodes[level]);
                 if (path->slots[level] + 1 >=
                     btrfs_header_nritems(path->nodes[level]))
                         continue;
@@ -1158,7 +1156,8 @@ int lookup_inline_extent_backref(struct btrfs_trans_handle *trans,
                  * For simplicity, we just do not add new inline back
                  * ref if there is any kind of item for this block
                  */
-                if (find_next_key(path, &key) == 0 && key.objectid == bytenr &&
+                if (find_next_key(path, 0, &key) == 0 &&
+                    key.objectid == bytenr &&
                     key.type < BTRFS_BLOCK_GROUP_ITEM_KEY) {
                         err = -EAGAIN;
                         goto out;
@@ -4128,6 +4127,7 @@ struct extent_buffer *btrfs_alloc_free_block(struct btrfs_trans_handle *trans,
         return buf;
 }
 
+#if 0
 int btrfs_drop_leaf_ref(struct btrfs_trans_handle *trans,
                         struct btrfs_root *root, struct extent_buffer *leaf)
 {
@@ -4171,8 +4171,6 @@ int btrfs_drop_leaf_ref(struct btrfs_trans_handle *trans,
         return 0;
 }
 
-#if 0
-
 static noinline int cache_drop_leaf_ref(struct btrfs_trans_handle *trans,
                                         struct btrfs_root *root,
                                         struct btrfs_leaf_ref *ref)
@@ -4553,262 +4551,471 @@ out:
 }
 #endif
 
+struct walk_control {
+        u64 refs[BTRFS_MAX_LEVEL];
+        u64 flags[BTRFS_MAX_LEVEL];
+        struct btrfs_key update_progress;
+        int stage;
+        int level;
+        int shared_level;
+        int update_ref;
+        int keep_locks;
+};
+
+#define DROP_REFERENCE  1
+#define UPDATE_BACKREF  2
+
 /*
- * helper function for drop_subtree, this function is similar to
- * walk_down_tree. The main difference is that it checks reference
- * counts while tree blocks are locked.
+ * hepler to process tree block while walking down the tree.
+ *
+ * when wc->stage == DROP_REFERENCE, this function checks
+ * reference count of the block. if the block is shared and
+ * we need update back refs for the subtree rooted at the
+ * block, this function changes wc->stage to UPDATE_BACKREF
+ *
+ * when wc->stage == UPDATE_BACKREF, this function updates
+ * back refs for pointers in the block.
+ *
+ * NOTE: return value 1 means we should stop walking down.
  */
-static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
+static noinline int walk_down_proc(struct btrfs_trans_handle *trans,
                                    struct btrfs_root *root,
-                                   struct btrfs_path *path, int *level)
+                                   struct btrfs_path *path,
+                                   struct walk_control *wc)
 {
-        struct extent_buffer *next;
-        struct extent_buffer *cur;
-        struct extent_buffer *parent;
-        u64 bytenr;
-        u64 ptr_gen;
-        u64 refs;
-        u64 flags;
-        u32 blocksize;
+        int level = wc->level;
+        struct extent_buffer *eb = path->nodes[level];
+        struct btrfs_key key;
+        u64 flag = BTRFS_BLOCK_FLAG_FULL_BACKREF;
         int ret;
 
-        cur = path->nodes[*level];
-        ret = btrfs_lookup_extent_info(trans, root, cur->start, cur->len,
-                                       &refs, &flags);
-        BUG_ON(ret);
-        if (refs > 1)
-                goto out;
+        if (wc->stage == UPDATE_BACKREF &&
+            btrfs_header_owner(eb) != root->root_key.objectid)
+                return 1;
 
-        BUG_ON(!(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF));
+        /*
+         * when reference count of tree block is 1, it won't increase
+         * again. once full backref flag is set, we never clear it.
+         */
+        if ((wc->stage == DROP_REFERENCE && wc->refs[level] != 1) ||
+            (wc->stage == UPDATE_BACKREF && !(wc->flags[level] & flag))) {
+                BUG_ON(!path->locks[level]);
+                ret = btrfs_lookup_extent_info(trans, root,
+                                               eb->start, eb->len,
+                                               &wc->refs[level],
+                                               &wc->flags[level]);
+                BUG_ON(ret);
+                BUG_ON(wc->refs[level] == 0);
+        }
 
-        while (*level >= 0) {
-                cur = path->nodes[*level];
-                if (*level == 0) {
-                        ret = btrfs_drop_leaf_ref(trans, root, cur);
-                        BUG_ON(ret);
-                        clean_tree_block(trans, root, cur);
-                        break;
-                }
-                if (path->slots[*level] >= btrfs_header_nritems(cur)) {
-                        clean_tree_block(trans, root, cur);
-                        break;
+        if (wc->stage == DROP_REFERENCE &&
+            wc->update_ref && wc->refs[level] > 1) {
+                BUG_ON(eb == root->node);
+                BUG_ON(path->slots[level] > 0);
+                if (level == 0)
+                        btrfs_item_key_to_cpu(eb, &key, path->slots[level]);
+                else
+                        btrfs_node_key_to_cpu(eb, &key, path->slots[level]);
+                if (btrfs_header_owner(eb) == root->root_key.objectid &&
+                    btrfs_comp_cpu_keys(&key, &wc->update_progress) >= 0) {
+                        wc->stage = UPDATE_BACKREF;
+                        wc->shared_level = level;
                 }
+        }
 
-                bytenr = btrfs_node_blockptr(cur, path->slots[*level]);
-                blocksize = btrfs_level_size(root, *level - 1);
-                ptr_gen = btrfs_node_ptr_generation(cur, path->slots[*level]);
+        if (wc->stage == DROP_REFERENCE) {
+                if (wc->refs[level] > 1)
+                        return 1;
 
-                next = read_tree_block(root, bytenr, blocksize, ptr_gen);
-                btrfs_tree_lock(next);
-                btrfs_set_lock_blocking(next);
+                if (path->locks[level] && !wc->keep_locks) {
+                        btrfs_tree_unlock(eb);
+                        path->locks[level] = 0;
+                }
+                return 0;
+        }
 
-                ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
-                                               &refs, &flags);
+        /* wc->stage == UPDATE_BACKREF */
+        if (!(wc->flags[level] & flag)) {
+                BUG_ON(!path->locks[level]);
+                ret = btrfs_inc_ref(trans, root, eb, 1);
                 BUG_ON(ret);
-                if (refs > 1) {
-                        parent = path->nodes[*level];
-                        ret = btrfs_free_extent(trans, root, bytenr,
-                                                blocksize, parent->start,
-                                                btrfs_header_owner(parent),
-                                                *level - 1, 0);
+                ret = btrfs_dec_ref(trans, root, eb, 0);
+                BUG_ON(ret);
+                ret = btrfs_set_disk_extent_flags(trans, root, eb->start,
+                                                  eb->len, flag, 0);
+                BUG_ON(ret);
+                wc->flags[level] |= flag;
+        }
+
+        /*
+         * the block is shared by multiple trees, so it's not good to
+         * keep the tree lock
+         */
+        if (path->locks[level] && level > 0) {
+                btrfs_tree_unlock(eb);
+                path->locks[level] = 0;
+        }
+        return 0;
+}
+
+/*
+ * hepler to process tree block while walking up the tree.
+ *
+ * when wc->stage == DROP_REFERENCE, this function drops
+ * reference count on the block.
+ *
+ * when wc->stage == UPDATE_BACKREF, this function changes
+ * wc->stage back to DROP_REFERENCE if we changed wc->stage
+ * to UPDATE_BACKREF previously while processing the block.
+ *
+ * NOTE: return value 1 means we should stop walking up.
+ */
+static noinline int walk_up_proc(struct btrfs_trans_handle *trans,
+                                 struct btrfs_root *root,
+                                 struct btrfs_path *path,
+                                 struct walk_control *wc)
+{
+        int ret = 0;
+        int level = wc->level;
+        struct extent_buffer *eb = path->nodes[level];
+        u64 parent = 0;
+
+        if (wc->stage == UPDATE_BACKREF) {
+                BUG_ON(wc->shared_level < level);
+                if (level < wc->shared_level)
+                        goto out;
+
+                BUG_ON(wc->refs[level] <= 1);
+                ret = find_next_key(path, level + 1, &wc->update_progress);
+                if (ret > 0)
+                        wc->update_ref = 0;
+
+                wc->stage = DROP_REFERENCE;
+                wc->shared_level = -1;
+                path->slots[level] = 0;
+
+                /*
+                 * check reference count again if the block isn't locked.
+                 * we should start walking down the tree again if reference
+                 * count is one.
+                 */
+                if (!path->locks[level]) {
+                        BUG_ON(level == 0);
+                        btrfs_tree_lock(eb);
+                        btrfs_set_lock_blocking(eb);
+                        path->locks[level] = 1;
+
+                        ret = btrfs_lookup_extent_info(trans, root,
+                                                       eb->start, eb->len,
+                                                       &wc->refs[level],
+                                                       &wc->flags[level]);
                         BUG_ON(ret);
-                        path->slots[*level]++;
-                        btrfs_tree_unlock(next);
-                        free_extent_buffer(next);
-                        continue;
+                        BUG_ON(wc->refs[level] == 0);
+                        if (wc->refs[level] == 1) {
+                                btrfs_tree_unlock(eb);
+                                path->locks[level] = 0;
+                                return 1;
+                        }
+                } else {
+                        BUG_ON(level != 0);
                 }
+        }
 
-                BUG_ON(!(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF));
+        /* wc->stage == DROP_REFERENCE */
+        BUG_ON(wc->refs[level] > 1 && !path->locks[level]);
 
-                *level = btrfs_header_level(next);
-                path->nodes[*level] = next;
-                path->slots[*level] = 0;
-                path->locks[*level] = 1;
-                cond_resched();
+        if (wc->refs[level] == 1) {
+                if (level == 0) {
+                        if (wc->flags[level] & BTRFS_BLOCK_FLAG_FULL_BACKREF)
+                                ret = btrfs_dec_ref(trans, root, eb, 1);
+                        else
+                                ret = btrfs_dec_ref(trans, root, eb, 0);
+                        BUG_ON(ret);
+                }
+                /* make block locked assertion in clean_tree_block happy */
+                if (!path->locks[level] &&
+                    btrfs_header_generation(eb) == trans->transid) {
+                        btrfs_tree_lock(eb);
+                        btrfs_set_lock_blocking(eb);
+                        path->locks[level] = 1;
+                }
+                clean_tree_block(trans, root, eb);
+        }
+
+        if (eb == root->node) {
+                if (wc->flags[level] & BTRFS_BLOCK_FLAG_FULL_BACKREF)
+                        parent = eb->start;
+                else
+                        BUG_ON(root->root_key.objectid !=
+                               btrfs_header_owner(eb));
+        } else {
+                if (wc->flags[level + 1] & BTRFS_BLOCK_FLAG_FULL_BACKREF)
+                        parent = path->nodes[level + 1]->start;
+                else
+                        BUG_ON(root->root_key.objectid !=
+                               btrfs_header_owner(path->nodes[level + 1]));
         }
-out:
-        if (path->nodes[*level] == root->node)
-                parent = path->nodes[*level];
-        else
-                parent = path->nodes[*level + 1];
-        bytenr = path->nodes[*level]->start;
-        blocksize = path->nodes[*level]->len;
 
-        ret = btrfs_free_extent(trans, root, bytenr, blocksize, parent->start,
-                                btrfs_header_owner(parent), *level, 0);
+        ret = btrfs_free_extent(trans, root, eb->start, eb->len, parent,
+                                root->root_key.objectid, level, 0);
         BUG_ON(ret);
+out:
+        wc->refs[level] = 0;
+        wc->flags[level] = 0;
+        return ret;
+}
+
+static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
+                                   struct btrfs_root *root,
+                                   struct btrfs_path *path,
+                                   struct walk_control *wc)
+{
+        struct extent_buffer *next;
+        struct extent_buffer *cur;
+        u64 bytenr;
+        u64 ptr_gen;
+        u32 blocksize;
+        int level = wc->level;
+        int ret;
+
+        while (level >= 0) {
+                cur = path->nodes[level];
+                BUG_ON(path->slots[level] >= btrfs_header_nritems(cur));
 
-        if (path->locks[*level]) {
-                btrfs_tree_unlock(path->nodes[*level]);
-                path->locks[*level] = 0;
+                ret = walk_down_proc(trans, root, path, wc);
+                if (ret > 0)
+                        break;
+
+                if (level == 0)
+                        break;
+
+                bytenr = btrfs_node_blockptr(cur, path->slots[level]);
+                blocksize = btrfs_level_size(root, level - 1);
+                ptr_gen = btrfs_node_ptr_generation(cur, path->slots[level]);
+
+                next = read_tree_block(root, bytenr, blocksize, ptr_gen);
+                btrfs_tree_lock(next);
+                btrfs_set_lock_blocking(next);
+
+                level--;
+                BUG_ON(level != btrfs_header_level(next));
+                path->nodes[level] = next;
+                path->slots[level] = 0;
+                path->locks[level] = 1;
+                wc->level = level;
         }
-        free_extent_buffer(path->nodes[*level]);
-        path->nodes[*level] = NULL;
-        *level += 1;
-        cond_resched();
         return 0;
 }
 
-/*
- * helper for dropping snapshots.  This walks back up the tree in the path
- * to find the first node higher up where we haven't yet gone through
- * all the slots
- */
 static noinline int walk_up_tree(struct btrfs_trans_handle *trans,
                                  struct btrfs_root *root,
                                  struct btrfs_path *path,
-                                 int *level, int max_level)
+                                 struct walk_control *wc, int max_level)
 {
-        struct btrfs_root_item *root_item = &root->root_item;
-        int i;
-        int slot;
+        int level = wc->level;
         int ret;
 
-        for (i = *level; i < max_level && path->nodes[i]; i++) {
-                slot = path->slots[i];
-                if (slot + 1 < btrfs_header_nritems(path->nodes[i])) {
-                        /*
-                         * there is more work to do in this level.
-                         * Update the drop_progress marker to reflect
-                         * the work we've done so far, and then bump
-                         * the slot number
-                         */
-                        path->slots[i]++;
-                        WARN_ON(*level == 0);
-                        if (max_level == BTRFS_MAX_LEVEL) {
-                                btrfs_node_key(path->nodes[i],
-                                               &root_item->drop_progress,
-                                               path->slots[i]);
-                                root_item->drop_level = i;
-                        }
-                        *level = i;
+        path->slots[level] = btrfs_header_nritems(path->nodes[level]);
+        while (level < max_level && path->nodes[level]) {
+                wc->level = level;
+                if (path->slots[level] + 1 <
+                    btrfs_header_nritems(path->nodes[level])) {
+                        path->slots[level]++;
                         return 0;
                 } else {
-                        struct extent_buffer *parent;
-
-                        /*
-                         * this whole node is done, free our reference
-                         * on it and go up one level
-                         */
-                        if (path->nodes[*level] == root->node)
-                                parent = path->nodes[*level];
-                        else
-                                parent = path->nodes[*level + 1];
+                        ret = walk_up_proc(trans, root, path, wc);
+                        if (ret > 0)
+                                return 0;
 
-                        clean_tree_block(trans, root, path->nodes[i]);
-                        ret = btrfs_free_extent(trans, root,
-                                                path->nodes[i]->start,
-                                                path->nodes[i]->len,
-                                                parent->start,
-                                                btrfs_header_owner(parent),
-                                                *level, 0);
-                        BUG_ON(ret);
-                        if (path->locks[*level]) {
-                                btrfs_tree_unlock(path->nodes[i]);
-                                path->locks[i] = 0;
+                        if (path->locks[level]) {
+                                btrfs_tree_unlock(path->nodes[level]);
+                                path->locks[level] = 0;
                         }
-                        free_extent_buffer(path->nodes[i]);
-                        path->nodes[i] = NULL;
-                        *level = i + 1;
+                        free_extent_buffer(path->nodes[level]);
+                        path->nodes[level] = NULL;
+                        level++;
                 }
         }
         return 1;
 }
 
 /*
- * drop the reference count on the tree rooted at 'snap'.  This traverses
- * the tree freeing any blocks that have a ref count of zero after being
- * decremented.
+ * drop a subvolume tree.
+ *
+ * this function traverses the tree freeing any blocks that only
+ * referenced by the tree.
+ *
+ * when a shared tree block is found. this function decreases its
+ * reference count by one. if update_ref is true, this function
+ * also make sure backrefs for the shared block and all lower level
+ * blocks are properly updated.
  */
-int btrfs_drop_snapshot(struct btrfs_trans_handle *trans, struct btrfs_root
-                        *root)
+int btrfs_drop_snapshot(struct btrfs_root *root, int update_ref)
 {
-        int ret = 0;
-        int wret;
-        int level;
         struct btrfs_path *path;
-        int update_count;
+        struct btrfs_trans_handle *trans;
+        struct btrfs_root *tree_root = root->fs_info->tree_root;
         struct btrfs_root_item *root_item = &root->root_item;
+        struct walk_control *wc;
+        struct btrfs_key key;
+        int err = 0;
+        int ret;
+        int level;
 
         path = btrfs_alloc_path();
         BUG_ON(!path);
 
-        level = btrfs_header_level(root->node);
+        wc = kzalloc(sizeof(*wc), GFP_NOFS);
+        BUG_ON(!wc);
+
+        trans = btrfs_start_transaction(tree_root, 1);
+
         if (btrfs_disk_key_objectid(&root_item->drop_progress) == 0) {
+                level = btrfs_header_level(root->node);
                 path->nodes[level] = btrfs_lock_root_node(root);
                 btrfs_set_lock_blocking(path->nodes[level]);
                 path->slots[level] = 0;
                 path->locks[level] = 1;
+                memset(&wc->update_progress, 0,
+                       sizeof(wc->update_progress));
         } else {
-                struct btrfs_key key;
-                struct btrfs_disk_key found_key;
-                struct extent_buffer *node;
-
                 btrfs_disk_key_to_cpu(&key, &root_item->drop_progress);
+                memcpy(&wc->update_progress, &key,
+                       sizeof(wc->update_progress));
+
                 level = root_item->drop_level;
+                BUG_ON(level == 0);
                 path->lowest_level = level;
-                wret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
-                if (wret < 0) {
-                        ret = wret;
+                ret = btrfs_search_slot(NULL, root, &key, path, 0, 0);
+                path->lowest_level = 0;
+                if (ret < 0) {
+                        err = ret;
                         goto out;
                 }
-                node = path->nodes[level];
-                btrfs_node_key(node, &found_key, path->slots[level]);
-                WARN_ON(memcmp(&found_key, &root_item->drop_progress,
-                               sizeof(found_key)));
+                btrfs_node_key_to_cpu(path->nodes[level], &key,
+                                      path->slots[level]);
+                WARN_ON(memcmp(&key, &wc->update_progress, sizeof(key)));
+
                 /*
                  * unlock our path, this is safe because only this
                  * function is allowed to delete this snapshot
                  */
                 btrfs_unlock_up_safe(path, 0);
+
+                level = btrfs_header_level(root->node);
+                while (1) {
+                        btrfs_tree_lock(path->nodes[level]);
+                        btrfs_set_lock_blocking(path->nodes[level]);
+
+                        ret = btrfs_lookup_extent_info(trans, root,
+                                                path->nodes[level]->start,
+                                                path->nodes[level]->len,
+                                                &wc->refs[level],
+                                                &wc->flags[level]);
+                        BUG_ON(ret);
+                        BUG_ON(wc->refs[level] == 0);
+
+                        if (level == root_item->drop_level)
+                                break;
+
+                        btrfs_tree_unlock(path->nodes[level]);
+                        WARN_ON(wc->refs[level] != 1);
+                        level--;
+                }
         }
+
+        wc->level = level;
+        wc->shared_level = -1;
+        wc->stage = DROP_REFERENCE;
+        wc->update_ref = update_ref;
+        wc->keep_locks = 0;
+
         while (1) {
-                unsigned long update;
-                wret = walk_down_tree(trans, root, path, &level);
-                if (wret > 0)
+                ret = walk_down_tree(trans, root, path, wc);
+                if (ret < 0) {
+                        err = ret;
                         break;
-                if (wret < 0)
-                        ret = wret;
+                }
 
-                wret = walk_up_tree(trans, root, path, &level,
-                                    BTRFS_MAX_LEVEL);
-                if (wret > 0)
+                ret = walk_up_tree(trans, root, path, wc, BTRFS_MAX_LEVEL);
+                if (ret < 0) {
+                        err = ret;
                         break;
-                if (wret < 0)
-                        ret = wret;
-                if (trans->transaction->in_commit ||
-                    trans->transaction->delayed_refs.flushing) {
-                        ret = -EAGAIN;
+                }
+
+                if (ret > 0) {
+                        BUG_ON(wc->stage != DROP_REFERENCE);
                         break;
                 }
-                for (update_count = 0; update_count < 16; update_count++) {
+
+                if (wc->stage == DROP_REFERENCE) {
+                        level = wc->level;
+                        btrfs_node_key(path->nodes[level],
+                                       &root_item->drop_progress,
+                                       path->slots[level]);
+                        root_item->drop_level = level;
+                }
+
+                BUG_ON(wc->level == 0);
+                if (trans->transaction->in_commit ||
+                    trans->transaction->delayed_refs.flushing) {
+                        ret = btrfs_update_root(trans, tree_root,
+                                                &root->root_key,
+                                                root_item);
+                        BUG_ON(ret);
+
+                        btrfs_end_transaction(trans, tree_root);
+                        trans = btrfs_start_transaction(tree_root, 1);
+                } else {
+                        unsigned long update;
                         update = trans->delayed_ref_updates;
                         trans->delayed_ref_updates = 0;
                         if (update)
-                                btrfs_run_delayed_refs(trans, root, update);
-                        else
-                                break;
+                                btrfs_run_delayed_refs(trans, tree_root,
+                                                       update);
                 }
         }
+        btrfs_release_path(root, path);
+        BUG_ON(err);
+
+        ret = btrfs_del_root(trans, tree_root, &root->root_key);
+        BUG_ON(ret);
+
+        free_extent_buffer(root->node);
+        free_extent_buffer(root->commit_root);
+        kfree(root);
 out:
+        btrfs_end_transaction(trans, tree_root);
+        kfree(wc);
         btrfs_free_path(path);
-        return ret;
+        return err;
 }
 
+/*
+ * drop subtree rooted at tree block 'node'.
+ *
+ * NOTE: this function will unlock and release tree block 'node'
+ */
 int btrfs_drop_subtree(struct btrfs_trans_handle *trans,
                         struct btrfs_root *root,
                         struct extent_buffer *node,
                         struct extent_buffer *parent)
 {
         struct btrfs_path *path;
+        struct walk_control *wc;
         int level;
         int parent_level;
         int ret = 0;
         int wret;
 
+        BUG_ON(root->root_key.objectid != BTRFS_TREE_RELOC_OBJECTID);
+
         path = btrfs_alloc_path();
         BUG_ON(!path);
 
+        wc = kzalloc(sizeof(*wc), GFP_NOFS);
+        BUG_ON(!wc);
+
         btrfs_assert_tree_locked(parent);
         parent_level = btrfs_header_level(parent);
         extent_buffer_get(parent);
@@ -4817,24 +5024,33 @@ int btrfs_drop_subtree(struct btrfs_trans_handle *trans,
 
         btrfs_assert_tree_locked(node);
         level = btrfs_header_level(node);
-        extent_buffer_get(node);
         path->nodes[level] = node;
         path->slots[level] = 0;
+        path->locks[level] = 1;
+
+        wc->refs[parent_level] = 1;
+        wc->flags[parent_level] = BTRFS_BLOCK_FLAG_FULL_BACKREF;
+        wc->level = level;
+        wc->shared_level = -1;
+        wc->stage = DROP_REFERENCE;
+        wc->update_ref = 0;
+        wc->keep_locks = 1;
 
         while (1) {
-                wret = walk_down_tree(trans, root, path, &level);
-                if (wret < 0)
+                wret = walk_down_tree(trans, root, path, wc);
+                if (wret < 0) {
                         ret = wret;
-                if (wret != 0)
                         break;
+                }
 
-                wret = walk_up_tree(trans, root, path, &level, parent_level);
+                wret = walk_up_tree(trans, root, path, wc, parent_level);
                 if (wret < 0)
                         ret = wret;
                 if (wret != 0)
                         break;
         }
 
+        kfree(wc);
         btrfs_free_path(path);
         return ret;
 }