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btrfs: avoid unnecessary root node COW during snapshotting

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There's no need to COW the root node of the subvolume we are snapshotting
because we then call btrfs_copy_root(), which creates a copy of the root
node and sets its generation to the current transaction. So remove this
redundant COW right before calling btrfs_copy_root(), saving one extent
allocation, memory allocation, copying things, etc, and making the code
less confusing. Also rename the extent buffer variable from "old" to
"root_eb" since that name no longer makes any sense after removing the
unnecessary COW operation.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
This commit is contained in:
Filipe Manana 2026-02-18 15:07:09 +00:00 committed by David Sterba
parent 13816fd5aa
commit 1fce8eec9f
1 changed files with 5 additions and 15 deletions
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20
fs/btrfs/transaction.c
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@ -1669,7 +1669,7 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
BTRFS_PATH_AUTO_FREE(path);
struct btrfs_dir_item *dir_item;
struct extent_buffer *tmp;
struct extent_buffer *old;
struct extent_buffer *root_eb;
struct timespec64 cur_time;
int ret = 0;
u64 to_reserve = 0;
@ -1807,20 +1807,10 @@ static noinline int create_pending_snapshot(struct btrfs_trans_handle *trans,
btrfs_set_stack_timespec_nsec(&new_root_item->otime, cur_time.tv_nsec);
btrfs_set_root_otransid(new_root_item, trans->transid);
old = btrfs_lock_root_node(root);
ret = btrfs_cow_block(trans, root, old, NULL, 0, &old,
BTRFS_NESTING_COW);
if (unlikely(ret)) {
btrfs_tree_unlock(old);
free_extent_buffer(old);
btrfs_abort_transaction(trans, ret);
goto fail;
}
ret = btrfs_copy_root(trans, root, old, &tmp, objectid);
/* clean up in any case */
btrfs_tree_unlock(old);
free_extent_buffer(old);
root_eb = btrfs_lock_root_node(root);
ret = btrfs_copy_root(trans, root, root_eb, &tmp, objectid);
btrfs_tree_unlock(root_eb);
free_extent_buffer(root_eb);
if (unlikely(ret)) {
btrfs_abort_transaction(trans, ret);
goto fail;