Re: [PATCH v4] btrfs: handle bio_split() error

[David Sterba <dsterba@suse.cz>]

On Thu, Nov 28, 2024 at 10:42:01AM +0100, Johannes Thumshirn wrote:
> From: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> 
> Commit bfebde92bd31 ("block: Rework bio_split() return value") changed
> bio_split() so that it can return errors.
> 
> Add error handling for it in btrfs_split_bio() and ultimately
> btrfs_submit_chunk().
> 
> Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
> ---
> 
> Changes to v3:
> - decrement bio counter

Is it possible to have some sanity checks for the bio counter? The
missing decrement could be hard to spot, but as it's a global counter
and possibly spanning unbounded time we'd have to add some checkpoints
like do per transaction accounting.

Otherwise,
Reviewed-by: David Sterba <dsterba@suse.com>

> Changes to v2:
> - assign the split bbio to a new variable, so we can keep the old error
>   paths and end the original bbio
> 
> Changes to v1:
> - convert ERR_PTR to blk_status_t
> - correctly fail already split bbios
> ---
>  fs/btrfs/bio.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/btrfs/bio.c b/fs/btrfs/bio.c
> index 1f216d07eff6..af3db0a7ae4d 100644
> --- a/fs/btrfs/bio.c
> +++ b/fs/btrfs/bio.c
> @@ -81,6 +81,9 @@ static struct btrfs_bio *btrfs_split_bio(struct btrfs_fs_info *fs_info,
>  
>  	bio = bio_split(&orig_bbio->bio, map_length >> SECTOR_SHIFT, GFP_NOFS,
>  			&btrfs_clone_bioset);
> +	if (IS_ERR(bio))
> +		return ERR_CAST(bio);

The cast is not strictly needed here as the pointer matches the return
type.

> +
>  	bbio = btrfs_bio(bio);
>  	btrfs_bio_init(bbio, fs_info, NULL, orig_bbio);
>  	bbio->inode = orig_bbio->inode;
> @@ -678,7 +681,8 @@ static bool btrfs_submit_chunk(struct btrfs_bio *bbio, int mirror_num)
>  				&bioc, &smap, &mirror_num);
>  	if (error) {
>  		ret = errno_to_blk_status(error);
> -		goto fail;
> +		btrfs_bio_counter_dec(fs_info);
> +		goto end_bbio;
>  	}
>  
>  	map_length = min(map_length, length);