btrfs: avoid checking for RO block group twice during nocow writeback

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.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Josef Bacik <josef@toxicpanda.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 535abf8..d6f0e1a 100644 (file)
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -1674,9 +1674,6 @@ next_slot:
                         */
                        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);
@@ -1723,6 +1720,7 @@ next_slot:
                                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;