This patch contains two changes to avoid unnecessary tree block reads during
snapshot dropping. First, check tree block''s reference count and flags
before
reading the tree block. if reference count > 1 and there is no need to update
backrefs, we can avoid reading the tree block. Second, save when snapshot was
created in root_key.offset. we can compare block pointer''s generation
with
snapshot''s creation generation during updating backrefs. If a given
block was
created before snapshot was created, the snapshot can''t be the tree
block''s
owner. So we can avoid reading the block.
Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
---
diff -urp 1/fs/btrfs/extent-tree.c 2/fs/btrfs/extent-tree.c
--- 1/fs/btrfs/extent-tree.c 2009-08-11 09:49:15.643738232 +0800
+++ 2/fs/btrfs/extent-tree.c 2009-08-24 10:36:43.016348000 +0800
@@ -4779,19 +4779,90 @@ struct walk_control {
int shared_level;
int update_ref;
int keep_locks;
+ int reada_end;
+ int reada_count;
};
#define DROP_REFERENCE 1
#define UPDATE_BACKREF 2
+static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct walk_control *wc,
+ struct btrfs_path *path)
+{
+ u64 bytenr;
+ u64 generation;
+ u64 refs;
+ u64 last = 0;
+ u32 nritems;
+ u32 blocksize;
+ struct btrfs_key key;
+ struct extent_buffer *eb;
+ int ret;
+ int slot;
+ int nread = 0;
+
+ if (path->slots[wc->level] < wc->reada_end) {
+ wc->reada_count = wc->reada_count * 2 / 3;
+ wc->reada_count = max(wc->reada_count, 2);
+ } else {
+ wc->reada_count = wc->reada_count * 3 / 2;
+ wc->reada_count = min_t(int, wc->reada_count,
+ BTRFS_NODEPTRS_PER_BLOCK(root));
+ }
+
+ eb = path->nodes[wc->level];
+ nritems = btrfs_header_nritems(eb);
+ blocksize = btrfs_level_size(root, wc->level - 1);
+
+ for (slot = path->slots[wc->level]; slot < nritems; slot++) {
+ if (nread >= wc->reada_count)
+ break;
+
+ cond_resched();
+ bytenr = btrfs_node_blockptr(eb, slot);
+ generation = btrfs_node_ptr_generation(eb, slot);
+
+ if (slot == path->slots[wc->level])
+ goto reada;
+
+ if (wc->stage == UPDATE_BACKREF &&
+ generation <= root->root_key.offset)
+ continue;
+
+ if (wc->stage == DROP_REFERENCE) {
+ ret = btrfs_lookup_extent_info(trans, root,
+ bytenr, blocksize,
+ &refs, NULL);
+ BUG_ON(ret);
+ BUG_ON(refs == 0);
+ if (refs == 1)
+ goto reada;
+
+ if (!wc->update_ref ||
+ generation <= root->root_key.offset)
+ continue;
+ btrfs_node_key_to_cpu(eb, &key, slot);
+ ret = btrfs_comp_cpu_keys(&key,
+ &wc->update_progress);
+ if (ret < 0)
+ continue;
+ }
+reada:
+ ret = readahead_tree_block(root, bytenr, blocksize,
+ generation);
+ if (ret)
+ break;
+ last = bytenr + blocksize;
+ nread++;
+ }
+ wc->reada_end = slot;
+}
+
/*
* 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.
*
@@ -4804,7 +4875,6 @@ static noinline int walk_down_proc(struc
{
int level = wc->level;
struct extent_buffer *eb = path->nodes[level];
- struct btrfs_key key;
u64 flag = BTRFS_BLOCK_FLAG_FULL_BACKREF;
int ret;
@@ -4827,21 +4897,6 @@ static noinline int walk_down_proc(struc
BUG_ON(wc->refs[level] == 0);
}
- 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;
- }
- }
-
if (wc->stage == DROP_REFERENCE) {
if (wc->refs[level] > 1)
return 1;
@@ -4878,6 +4933,123 @@ static noinline int walk_down_proc(struc
}
/*
+ * hepler to process tree block pointer.
+ *
+ * when wc->stage == DROP_REFERENCE, this function checks
+ * reference count of the block pointed to. 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. if the block is shared and there is no
+ * need to update back, this function drops the reference
+ * to the block.
+ *
+ * NOTE: return value 1 means we should stop walking down.
+ */
+static noinline int do_walk_down(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct btrfs_path *path,
+ struct walk_control *wc)
+{
+ u64 bytenr;
+ u64 generation;
+ u64 parent;
+ u32 blocksize;
+ struct btrfs_key key;
+ struct extent_buffer *next;
+ int level = wc->level;
+ int reada = 0;
+ int ret = 0;
+
+ generation = btrfs_node_ptr_generation(path->nodes[level],
+ path->slots[level]);
+ /*
+ * if the lower level block was created before the snapshot
+ * was created, we know there is no need to update back refs
+ * for the subtree
+ */
+ if (wc->stage == UPDATE_BACKREF &&
+ generation <= root->root_key.offset)
+ return 1;
+
+ bytenr = btrfs_node_blockptr(path->nodes[level], path->slots[level]);
+ blocksize = btrfs_level_size(root, level - 1);
+
+ next = btrfs_find_tree_block(root, bytenr, blocksize);
+ if (!next) {
+ next = btrfs_find_create_tree_block(root, bytenr, blocksize);
+ reada = 1;
+ }
+ btrfs_tree_lock(next);
+ btrfs_set_lock_blocking(next);
+
+ if (wc->stage == DROP_REFERENCE) {
+ ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
+ &wc->refs[level - 1],
+ &wc->flags[level - 1]);
+ BUG_ON(ret);
+ BUG_ON(wc->refs[level - 1] == 0);
+
+ if (wc->refs[level - 1] > 1) {
+ if (!wc->update_ref ||
+ generation <= root->root_key.offset)
+ goto skip;
+
+ btrfs_node_key_to_cpu(path->nodes[level], &key,
+ path->slots[level]);
+ ret = btrfs_comp_cpu_keys(&key, &wc->update_progress);
+ if (ret < 0)
+ goto skip;
+
+ wc->stage = UPDATE_BACKREF;
+ wc->shared_level = level - 1;
+ }
+ }
+
+ if (!btrfs_buffer_uptodate(next, generation)) {
+ btrfs_tree_unlock(next);
+ free_extent_buffer(next);
+ next = NULL;
+ }
+
+ if (!next) {
+ if (reada && level == 1)
+ reada_walk_down(trans, root, wc, path);
+ next = read_tree_block(root, bytenr, blocksize, generation);
+ 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;
+ if (wc->level == 1)
+ wc->reada_end = 0;
+ return 0;
+skip:
+ wc->refs[level - 1] = 0;
+ wc->flags[level - 1] = 0;
+
+ if (wc->flags[level] & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
+ parent = path->nodes[level]->start;
+ } else {
+ BUG_ON(root->root_key.objectid !+
btrfs_header_owner(path->nodes[level]));
+ parent = 0;
+ }
+
+ ret = btrfs_free_extent(trans, root, bytenr, blocksize, parent,
+ root->root_key.objectid, level - 1, 0);
+ BUG_ON(ret);
+
+ btrfs_tree_unlock(next);
+ free_extent_buffer(next);
+ return 1;
+}
+
+/*
* hepler to process tree block while walking up the tree.
*
* when wc->stage == DROP_REFERENCE, this function drops
@@ -4904,7 +5076,6 @@ static noinline int walk_up_proc(struct
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;
@@ -4935,8 +5106,6 @@ static noinline int walk_up_proc(struct
path->locks[level] = 0;
return 1;
}
- } else {
- BUG_ON(level != 0);
}
}
@@ -4989,17 +5158,13 @@ static noinline int walk_down_tree(struc
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->slots[level] >+
btrfs_header_nritems(path->nodes[level]))
+ break;
ret = walk_down_proc(trans, root, path, wc);
if (ret > 0)
@@ -5008,20 +5173,12 @@ static noinline int walk_down_tree(struc
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;
+ ret = do_walk_down(trans, root, path, wc);
+ if (ret > 0) {
+ path->slots[level]++;
+ continue;
+ }
+ level = wc->level;
}
return 0;
}
@@ -5111,9 +5268,7 @@ int btrfs_drop_snapshot(struct btrfs_roo
err = ret;
goto out;
}
- btrfs_node_key_to_cpu(path->nodes[level], &key,
- path->slots[level]);
- WARN_ON(memcmp(&key, &wc->update_progress, sizeof(key)));
+ WARN_ON(ret > 0);
/*
* unlock our path, this is safe because only this
@@ -5148,6 +5303,7 @@ int btrfs_drop_snapshot(struct btrfs_roo
wc->stage = DROP_REFERENCE;
wc->update_ref = update_ref;
wc->keep_locks = 0;
+ wc->reada_count = BTRFS_NODEPTRS_PER_BLOCK(root);
while (1) {
ret = walk_down_tree(trans, root, path, wc);
@@ -5254,6 +5410,7 @@ int btrfs_drop_subtree(struct btrfs_tran
wc->stage = DROP_REFERENCE;
wc->update_ref = 0;
wc->keep_locks = 1;
+ wc->reada_count = BTRFS_NODEPTRS_PER_BLOCK(root);
while (1) {
wret = walk_down_tree(trans, root, path, wc);
diff -urp 1/fs/btrfs/transaction.c 2/fs/btrfs/transaction.c
--- 1/fs/btrfs/transaction.c 2009-07-31 13:48:03.038025845 +0800
+++ 2/fs/btrfs/transaction.c 2009-08-24 10:36:46.248107000 +0800
@@ -720,7 +720,8 @@ static noinline int create_pending_snaps
memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
key.objectid = objectid;
- key.offset = 0;
+ /* record when the snapshot was created in key.offset */
+ key.offset = trans->transid;
btrfs_set_key_type(&key, BTRFS_ROOT_ITEM_KEY);
old = btrfs_lock_root_node(root);
--
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This patch contains two changes to avoid unnecessary tree block reads during
snapshot dropping. First, check tree block''s reference count and flags
before
reading the tree block. if reference count > 1 and there is no need to update
backrefs, we can avoid reading the tree block. Second, save when snapshot was
created in root_key.offset. we can compare block pointer''s generation
with
snapshot''s creation generation during updating backrefs. If a given
block was
created before snapshot was created, the snapshot can''t be the tree
block''s
owner. So we can avoid reading the block.
Signed-off-by: Yan Zheng <zheng.yan@oracle.com>
---
diff -urp 1/fs/btrfs/extent-tree.c 2/fs/btrfs/extent-tree.c
--- 1/fs/btrfs/extent-tree.c 2009-09-16 15:24:37.254170718 +0800
+++ 2/fs/btrfs/extent-tree.c 2009-09-18 15:00:46.433350551 +0800
@@ -4779,19 +4779,90 @@ struct walk_control {
int shared_level;
int update_ref;
int keep_locks;
+ int reada_slot;
+ int reada_count;
};
#define DROP_REFERENCE 1
#define UPDATE_BACKREF 2
+static noinline void reada_walk_down(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct walk_control *wc,
+ struct btrfs_path *path)
+{
+ u64 bytenr;
+ u64 generation;
+ u64 refs;
+ u64 last = 0;
+ u32 nritems;
+ u32 blocksize;
+ struct btrfs_key key;
+ struct extent_buffer *eb;
+ int ret;
+ int slot;
+ int nread = 0;
+
+ if (path->slots[wc->level] < wc->reada_slot) {
+ wc->reada_count = wc->reada_count * 2 / 3;
+ wc->reada_count = max(wc->reada_count, 2);
+ } else {
+ wc->reada_count = wc->reada_count * 3 / 2;
+ wc->reada_count = min_t(int, wc->reada_count,
+ BTRFS_NODEPTRS_PER_BLOCK(root));
+ }
+
+ eb = path->nodes[wc->level];
+ nritems = btrfs_header_nritems(eb);
+ blocksize = btrfs_level_size(root, wc->level - 1);
+
+ for (slot = path->slots[wc->level]; slot < nritems; slot++) {
+ if (nread >= wc->reada_count)
+ break;
+
+ cond_resched();
+ bytenr = btrfs_node_blockptr(eb, slot);
+ generation = btrfs_node_ptr_generation(eb, slot);
+
+ if (slot == path->slots[wc->level])
+ goto reada;
+
+ if (wc->stage == UPDATE_BACKREF &&
+ generation <= root->root_key.offset)
+ continue;
+
+ if (wc->stage == DROP_REFERENCE) {
+ ret = btrfs_lookup_extent_info(trans, root,
+ bytenr, blocksize,
+ &refs, NULL);
+ BUG_ON(ret);
+ BUG_ON(refs == 0);
+ if (refs == 1)
+ goto reada;
+
+ if (!wc->update_ref ||
+ generation <= root->root_key.offset)
+ continue;
+ btrfs_node_key_to_cpu(eb, &key, slot);
+ ret = btrfs_comp_cpu_keys(&key,
+ &wc->update_progress);
+ if (ret < 0)
+ continue;
+ }
+reada:
+ ret = readahead_tree_block(root, bytenr, blocksize,
+ generation);
+ if (ret)
+ break;
+ last = bytenr + blocksize;
+ nread++;
+ }
+ wc->reada_slot = slot;
+}
+
/*
* 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.
*
@@ -4804,7 +4875,6 @@ static noinline int walk_down_proc(struc
{
int level = wc->level;
struct extent_buffer *eb = path->nodes[level];
- struct btrfs_key key;
u64 flag = BTRFS_BLOCK_FLAG_FULL_BACKREF;
int ret;
@@ -4827,21 +4897,6 @@ static noinline int walk_down_proc(struc
BUG_ON(wc->refs[level] == 0);
}
- 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;
- }
- }
-
if (wc->stage == DROP_REFERENCE) {
if (wc->refs[level] > 1)
return 1;
@@ -4878,6 +4933,123 @@ static noinline int walk_down_proc(struc
}
/*
+ * hepler to process tree block pointer.
+ *
+ * when wc->stage == DROP_REFERENCE, this function checks
+ * reference count of the block pointed to. 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. if the block is shared and there is no
+ * need to update back, this function drops the reference
+ * to the block.
+ *
+ * NOTE: return value 1 means we should stop walking down.
+ */
+static noinline int do_walk_down(struct btrfs_trans_handle *trans,
+ struct btrfs_root *root,
+ struct btrfs_path *path,
+ struct walk_control *wc)
+{
+ u64 bytenr;
+ u64 generation;
+ u64 parent;
+ u32 blocksize;
+ struct btrfs_key key;
+ struct extent_buffer *next;
+ int level = wc->level;
+ int reada = 0;
+ int ret = 0;
+
+ generation = btrfs_node_ptr_generation(path->nodes[level],
+ path->slots[level]);
+ /*
+ * if the lower level block was created before the snapshot
+ * was created, we know there is no need to update back refs
+ * for the subtree
+ */
+ if (wc->stage == UPDATE_BACKREF &&
+ generation <= root->root_key.offset)
+ return 1;
+
+ bytenr = btrfs_node_blockptr(path->nodes[level], path->slots[level]);
+ blocksize = btrfs_level_size(root, level - 1);
+
+ next = btrfs_find_tree_block(root, bytenr, blocksize);
+ if (!next) {
+ next = btrfs_find_create_tree_block(root, bytenr, blocksize);
+ reada = 1;
+ }
+ btrfs_tree_lock(next);
+ btrfs_set_lock_blocking(next);
+
+ if (wc->stage == DROP_REFERENCE) {
+ ret = btrfs_lookup_extent_info(trans, root, bytenr, blocksize,
+ &wc->refs[level - 1],
+ &wc->flags[level - 1]);
+ BUG_ON(ret);
+ BUG_ON(wc->refs[level - 1] == 0);
+
+ if (wc->refs[level - 1] > 1) {
+ if (!wc->update_ref ||
+ generation <= root->root_key.offset)
+ goto skip;
+
+ btrfs_node_key_to_cpu(path->nodes[level], &key,
+ path->slots[level]);
+ ret = btrfs_comp_cpu_keys(&key, &wc->update_progress);
+ if (ret < 0)
+ goto skip;
+
+ wc->stage = UPDATE_BACKREF;
+ wc->shared_level = level - 1;
+ }
+ }
+
+ if (!btrfs_buffer_uptodate(next, generation)) {
+ btrfs_tree_unlock(next);
+ free_extent_buffer(next);
+ next = NULL;
+ }
+
+ if (!next) {
+ if (reada && level == 1)
+ reada_walk_down(trans, root, wc, path);
+ next = read_tree_block(root, bytenr, blocksize, generation);
+ 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;
+ if (wc->level == 1)
+ wc->reada_slot = 0;
+ return 0;
+skip:
+ wc->refs[level - 1] = 0;
+ wc->flags[level - 1] = 0;
+
+ if (wc->flags[level] & BTRFS_BLOCK_FLAG_FULL_BACKREF) {
+ parent = path->nodes[level]->start;
+ } else {
+ BUG_ON(root->root_key.objectid !+
btrfs_header_owner(path->nodes[level]));
+ parent = 0;
+ }
+
+ ret = btrfs_free_extent(trans, root, bytenr, blocksize, parent,
+ root->root_key.objectid, level - 1, 0);
+ BUG_ON(ret);
+
+ btrfs_tree_unlock(next);
+ free_extent_buffer(next);
+ return 1;
+}
+
+/*
* hepler to process tree block while walking up the tree.
*
* when wc->stage == DROP_REFERENCE, this function drops
@@ -4904,7 +5076,6 @@ static noinline int walk_up_proc(struct
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;
@@ -4935,8 +5106,6 @@ static noinline int walk_up_proc(struct
path->locks[level] = 0;
return 1;
}
- } else {
- BUG_ON(level != 0);
}
}
@@ -4989,17 +5158,13 @@ static noinline int walk_down_tree(struc
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->slots[level] >+
btrfs_header_nritems(path->nodes[level]))
+ break;
ret = walk_down_proc(trans, root, path, wc);
if (ret > 0)
@@ -5008,20 +5173,12 @@ static noinline int walk_down_tree(struc
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;
+ ret = do_walk_down(trans, root, path, wc);
+ if (ret > 0) {
+ path->slots[level]++;
+ continue;
+ }
+ level = wc->level;
}
return 0;
}
@@ -5111,9 +5268,7 @@ int btrfs_drop_snapshot(struct btrfs_roo
err = ret;
goto out;
}
- btrfs_node_key_to_cpu(path->nodes[level], &key,
- path->slots[level]);
- WARN_ON(memcmp(&key, &wc->update_progress, sizeof(key)));
+ WARN_ON(ret > 0);
/*
* unlock our path, this is safe because only this
@@ -5148,6 +5303,7 @@ int btrfs_drop_snapshot(struct btrfs_roo
wc->stage = DROP_REFERENCE;
wc->update_ref = update_ref;
wc->keep_locks = 0;
+ wc->reada_count = BTRFS_NODEPTRS_PER_BLOCK(root);
while (1) {
ret = walk_down_tree(trans, root, path, wc);
@@ -5254,6 +5410,7 @@ int btrfs_drop_subtree(struct btrfs_tran
wc->stage = DROP_REFERENCE;
wc->update_ref = 0;
wc->keep_locks = 1;
+ wc->reada_count = BTRFS_NODEPTRS_PER_BLOCK(root);
while (1) {
wret = walk_down_tree(trans, root, path, wc);
diff -urp 1/fs/btrfs/transaction.c 2/fs/btrfs/transaction.c
--- 1/fs/btrfs/transaction.c 2009-09-16 15:24:37.333171090 +0800
+++ 2/fs/btrfs/transaction.c 2009-09-18 15:00:46.435351323 +0800
@@ -720,7 +720,8 @@ static noinline int create_pending_snaps
memcpy(new_root_item, &root->root_item, sizeof(*new_root_item));
key.objectid = objectid;
- key.offset = 0;
+ /* record when the snapshot was created in key.offset */
+ key.offset = trans->transid;
btrfs_set_key_type(&key, BTRFS_ROOT_ITEM_KEY);
old = btrfs_lock_root_node(root);
--
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