[PATCH] btrfs: check snapshot_force_cow earlier in can_nocow_file_extent

[jkchen]

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.

This patch 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: jkchen <jkchen@xxxxxxxxxx>
---
 fs/btrfs/inode.c | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 845164485..3549031f3 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1921,6 +1921,11 @@ 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 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);
 
@@ -1982,11 +1987,6 @@ 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;
-- 
2.43.0