Re: [PATCH v2 1/3] btrfs: remove ffe RAID loop

[Boris Burkov <boris@bur.io>]

On Fri, Oct 03, 2025 at 04:41:57PM -0700, Leo Martins wrote:
> This patch removes the RAID loop from find_free_extent since it
> is impossible to allocate from a block group with a different
> RAID profile.
> 
> Historically, we've been able to fulfill allocation requests
> from mismatched RAID block groups assuming they provided the
> required duplcation. For example, a request for RAID0 could be
> fulfilled by a RAID1 block group.
> 
> 2a28468e525f ("btrfs: extent-tree: Make sure we only allocate extents from block groups with the same type")
> changed this behavior to skip block groups with different flags
> than the request. This makes the duplication compatiblity check
> redundant since we're going to keep searching regardless.
> 
> Signed-off-by: Leo Martins <loemra.dev@gmail.com>

Reviewed-by: Boris Burkov <boris@bur.io>

> ---
>  fs/btrfs/extent-tree.c | 32 +-------------------------------
>  1 file changed, 1 insertion(+), 31 deletions(-)
> 
> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
> index a4416c451b25..28b442660014 100644
> --- a/fs/btrfs/extent-tree.c
> +++ b/fs/btrfs/extent-tree.c
> @@ -4171,13 +4171,8 @@ static int find_free_extent_update_loop(struct btrfs_fs_info *fs_info,
>  	if (ffe_ctl->loop >= LOOP_CACHING_WAIT && ffe_ctl->have_caching_bg)
>  		return 1;
>  
> -	ffe_ctl->index++;
> -	if (ffe_ctl->index < BTRFS_NR_RAID_TYPES)
> -		return 1;
> -
>  	/* See the comments for btrfs_loop_type for an explanation of the phases. */
>  	if (ffe_ctl->loop < LOOP_NO_EMPTY_SIZE) {
> -		ffe_ctl->index = 0;
>  		/*
>  		 * We want to skip the LOOP_CACHING_WAIT step if we don't have
>  		 * any uncached bgs and we've already done a full search
> @@ -4477,9 +4472,7 @@ static noinline int find_free_extent(struct btrfs_root *root,
>  search:
>  	trace_btrfs_find_free_extent_search_loop(root, ffe_ctl);
>  	ffe_ctl->have_caching_bg = false;
> -	if (ffe_ctl->index == btrfs_bg_flags_to_raid_index(ffe_ctl->flags) ||
> -	    ffe_ctl->index == 0)
> -		full_search = true;
> +	full_search = true;
>  	down_read(&space_info->groups_sem);
>  	list_for_each_entry(block_group,
>  			    &space_info->block_groups[ffe_ctl->index], list) {
> @@ -4498,30 +4491,7 @@ static noinline int find_free_extent(struct btrfs_root *root,
>  		btrfs_grab_block_group(block_group, ffe_ctl->delalloc);
>  		ffe_ctl->search_start = block_group->start;
>  
> -		/*
> -		 * this can happen if we end up cycling through all the
> -		 * raid types, but we want to make sure we only allocate
> -		 * for the proper type.
> -		 */
>  		if (!block_group_bits(block_group, ffe_ctl->flags)) {
> -			u64 extra = BTRFS_BLOCK_GROUP_DUP |
> -				BTRFS_BLOCK_GROUP_RAID1_MASK |
> -				BTRFS_BLOCK_GROUP_RAID56_MASK |
> -				BTRFS_BLOCK_GROUP_RAID10;
> -
> -			/*
> -			 * if they asked for extra copies and this block group
> -			 * doesn't provide them, bail.  This does allow us to
> -			 * fill raid0 from raid1.
> -			 */
> -			if ((ffe_ctl->flags & extra) && !(block_group->flags & extra))
> -				goto loop;
> -
> -			/*
> -			 * This block group has different flags than we want.
> -			 * It's possible that we have MIXED_GROUP flag but no
> -			 * block group is mixed.  Just skip such block group.
> -			 */
>  			btrfs_release_block_group(block_group, ffe_ctl->delalloc);
>  			continue;
>  		}
> -- 
> 2.47.3
>