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 <jk.chen1095@gmail.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 2ae5a9b2f9511bf70786ca497f3d8c7d0d9eca42..f6ca6a76730e39eaf04e868fd4fa1030be403a8b 100644 (file)
--- 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;