[PATCH v2 1/3] btrfs: avoid checking for RO block group twice during nocow writeback

[fdmanana@kernel.org]

From: Filipe Manana <fdmanana@suse.com>

During the nocow writeback path, we currently iterate the rbtree of block
groups twice: once for checking if the target block group is RO with the
call to btrfs_extent_readonly()), and once again for getting a nocow
reference on the block group with a call to btrfs_inc_nocow_writers().

Since btrfs_inc_nocow_writers() already returns false when the target
block group is RO, remove the call to btrfs_extent_readonly(). Not only
we avoid searching the blocks group rbtree twice, it also helps reduce
contention on the lock that protects it (specially since it is a spin
lock and not a read-write lock). That may make a noticeable difference
on very large filesystems, with thousands of allocated block groups.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
---
 fs/btrfs/inode.c | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index 04cd95899ac8..76a0151ef05a 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1657,9 +1657,6 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
 			 */
 			btrfs_release_path(path);
 
-			/* If extent is RO, we must COW it */
-			if (btrfs_extent_readonly(fs_info, disk_bytenr))
-				goto out_check;
 			ret = btrfs_cross_ref_exist(root, ino,
 						    found_key.offset -
 						    extent_offset, disk_bytenr, false);
@@ -1706,6 +1703,7 @@ static noinline int run_delalloc_nocow(struct btrfs_inode *inode,
 				WARN_ON_ONCE(freespace_inode);
 				goto out_check;
 			}
+			/* If the extent's block group is RO, we must COW. */
 			if (!btrfs_inc_nocow_writers(fs_info, disk_bytenr))
 				goto out_check;
 			nocow = true;
-- 
2.28.0