From: Chen Guan Jie Date: Mon, 16 Feb 2026 22:16:32 +0000 (+0800) Subject: btrfs: check snapshot_force_cow earlier in can_nocow_file_extent() X-Git-Url: http://git.ipfire.org/index.cgi?a=commitdiff_plain;h=13816fd5aa3ca2842be5dba1dcff3b86b174c9c0;p=thirdparty%2Fkernel%2Flinux.git btrfs: check snapshot_force_cow earlier in can_nocow_file_extent() When a snapshot is being created, the atomic counter snapshot_force_cow is incremented to force incoming writes to fallback to COW. This is a critical mechanism to protect the consistency of the snapshot being taken. Currently, can_nocow_file_extent() checks this counter only after performing several checks, most notably the expensive cross-reference check via btrfs_cross_ref_exist(). btrfs_cross_ref_exist() releases the path and performs a search in the extent tree or backref cache, which involves btree traversals and locking overhead. Moves the snapshot_force_cow check to the very beginning of can_nocow_file_extent(). This reordering is safe and beneficial because: 1. args->writeback_path is invariant for the duration of the call (set by caller run_delalloc_nocow). 2. is_freespace_inode is a static property of the inode. 3. The state of snapshot_force_cow is driven by the btrfs_mksnapshot() process. Checking it earlier does not change the outcome of the NOCOW decision, but effectively prunes the expensive code path when a fallback to COW is inevitable. By failing fast when a snapshot is pending, we avoid the unnecessary overhead of btrfs_cross_ref_exist() and other extent item checks in the scenario where NOCOW is already known to be impossible. Signed-off-by: Chen Guan Jie Reviewed-by: Filipe Manana Signed-off-by: Filipe Manana Signed-off-by: David Sterba --- diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c index 2ae5a9b2f9511..f6ca6a76730e3 100644 --- a/fs/btrfs/inode.c +++ b/fs/btrfs/inode.c @@ -1943,6 +1943,11 @@ static int can_nocow_file_extent(struct btrfs_path *path, int ret = 0; bool nowait = path->nowait; + /* If there are pending snapshots for this root, we must do COW. */ + if (args->writeback_path && !is_freespace_inode && + atomic_read(&root->snapshot_force_cow)) + goto out; + fi = btrfs_item_ptr(leaf, path->slots[0], struct btrfs_file_extent_item); extent_type = btrfs_file_extent_type(leaf, fi); @@ -2004,11 +2009,6 @@ static int can_nocow_file_extent(struct btrfs_path *path, path = NULL; } - /* If there are pending snapshots for this root, we must COW. */ - if (args->writeback_path && !is_freespace_inode && - atomic_read(&root->snapshot_force_cow)) - goto out; - args->file_extent.num_bytes = min(args->end + 1, extent_end) - args->start; args->file_extent.offset += args->start - key->offset; io_start = args->file_extent.disk_bytenr + args->file_extent.offset;