Re: [PATCH V3] block: optimize for small block size IO

[Ming Lei <ming.lei@redhat.com>]

On Tue, Oct 29, 2019 at 04:04:25AM -0700, Christoph Hellwig wrote:
> I still haven't seen an explanation why this simple thing doesn't work
> just as well:
> 
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index 48e6725b32ee..f3073700166f 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -309,6 +309,11 @@ void __blk_queue_split(struct request_queue *q, struct bio **bio,
>  				nr_segs);
>  		break;
>  	default:
> +		if ((*bio)->bi_vcnt == 1 &&
> +		    (*bio)->bi_io_vec[0].bv_len <= PAGE_SIZE) {
> +			*nr_segs = 1;
> +			return;
> +		}
>  		split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
>  		break;
>  	}

This bio(*bio) may be a fast-cloned bio from somewhere(DM, MD, ...), so the above
check can't work sometime.


thanks,
Ming