Re: [PATCH 1/4] btrfs: avoid checking for RO block group twice during nocow writeback

[Anand Jain <anand.jain@oracle.com>]

On 2/3/2021 7:17 PM, fdmanana@kernel.org wrote:
> 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>

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Thanks.

> ---
>   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 589030cefd90..b10fc42f9e9a 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;
>