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Commit 2c47e605 authored by Yan Zheng's avatar Yan Zheng Committed by Chris Mason
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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: default avatarYan Zheng <zheng.yan@oracle.com>
Signed-off-by: default avatarChris Mason <chris.mason@oracle.com>
parent a970b0a1
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Inline Side-by-side
Showing with 395 additions and 181 deletions
fs/btrfs/ctree.h
View file @ 2c47e605
......@@ -2076,8 +2076,7 @@ static inline int btrfs_insert_empty_item(struct btrfs_trans_handle *trans,
int btrfs_next_leaf(struct btrfs_root *root, struct btrfs_path *path);
int btrfs_prev_leaf(struct btrfs_root *root, struct btrfs_path *path);
int btrfs_leaf_free_space(struct btrfs_root *root, struct extent_buffer *leaf);
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 btrfs_drop_subtree(struct btrfs_trans_handle *trans,
struct btrfs_root *root,
struct extent_buffer *node,
......
fs/btrfs/extent-tree.c
View file @ 2c47e605
......@@ -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 @@ static noinline int walk_down_tree(struct btrfs_trans_handle *trans,
}
#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);
if (wc->stage == UPDATE_BACKREF &&
btrfs_header_owner(eb) != root->root_key.objectid)
return 1;
/*
* 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);
if (refs > 1)
goto out;
BUG_ON(wc->refs[level] == 0);
}
BUG_ON(!(flags & BTRFS_BLOCK_FLAG_FULL_BACKREF));
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;
}
}
while (*level >= 0) {
cur = path->nodes[*level];
if (*level == 0) {
ret = btrfs_drop_leaf_ref(trans, root, cur);
if (wc->stage == DROP_REFERENCE) {
if (wc->refs[level] > 1)
return 1;
if (path->locks[level] && !wc->keep_locks) {
btrfs_tree_unlock(eb);
path->locks[level] = 0;
}
return 0;
}
/* 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);
clean_tree_block(trans, root, cur);
break;
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;
}
if (path->slots[*level] >= btrfs_header_nritems(cur)) {
clean_tree_block(trans, root, cur);
break;
/*
* 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;
}
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]);
/*
* 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;
next = read_tree_block(root, bytenr, blocksize, ptr_gen);
btrfs_tree_lock(next);
btrfs_set_lock_blocking(next);
if (wc->stage == UPDATE_BACKREF) {
BUG_ON(wc->shared_level < level);
if (level < wc->shared_level)
goto out;
ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
&refs, &flags);
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);
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);
}
out:
if (path->nodes[*level] == root->node)
parent = path->nodes[*level];
/* 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
parent = path->nodes[*level + 1];
bytenr = path->nodes[*level]->start;
blocksize = path->nodes[*level]->len;
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]));
}
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;
}
if (path->locks[*level]) {
btrfs_tree_unlock(path->nodes[*level]);
path->locks[*level] = 0;
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));
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;
}
......
fs/btrfs/relocation.c
View file @ 2c47e605
......@@ -1788,7 +1788,7 @@ static void merge_func(struct btrfs_work *work)
btrfs_end_transaction(trans, root);
}
btrfs_drop_dead_root(reloc_root);
btrfs_drop_snapshot(reloc_root, 0);
if (atomic_dec_and_test(async->num_pending))
complete(async->done);
......@@ -2075,9 +2075,6 @@ static int do_relocation(struct btrfs_trans_handle *trans,
ret = btrfs_drop_subtree(trans, root, eb, upper->eb);
BUG_ON(ret);
btrfs_tree_unlock(eb);
free_extent_buffer(eb);
}
if (!lowest) {
btrfs_tree_unlock(upper->eb);
......
fs/btrfs/transaction.c
View file @ 2c47e605
......@@ -593,6 +593,7 @@ int btrfs_defrag_root(struct btrfs_root *root, int cacheonly)
return 0;
}
#if 0
/*
* when dropping snapshots, we generate a ton of delayed refs, and it makes
* sense not to join the transaction while it is trying to flush the current
......@@ -681,6 +682,7 @@ int btrfs_drop_dead_root(struct btrfs_root *root)
btrfs_btree_balance_dirty(tree_root, nr);
return ret;
}
#endif
/*
* new snapshots need to be created at a very specific time in the
......@@ -1081,7 +1083,7 @@ int btrfs_clean_old_snapshots(struct btrfs_root *root)
while (!list_empty(&list)) {
root = list_entry(list.next, struct btrfs_root, root_list);
list_del_init(&root->root_list);
btrfs_drop_dead_root(root);
btrfs_drop_snapshot(root, 0);
}
return 0;
}
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