Re: [PATCH 1/4] block: bio: introduce helpers to get the 1st and last bvec

[Sagi Grimberg <sagig@dev.mellanox.co.il>]

> Cc Kent and Keith.
>
> Follows another version which should be more efficient.
> Kent and Keith, I appreciate much if you may give a review on it.
>
> diff --git a/include/linux/bio.h b/include/linux/bio.h
> index 56d2db8..ef45fec 100644
> --- a/include/linux/bio.h
> +++ b/include/linux/bio.h
> @@ -278,11 +278,21 @@ static inline void bio_get_first_bvec(struct bio *bio, struct bio_vec *bv)
>    */
>   static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
>   {
> -	struct bvec_iter iter;
> +	struct bvec_iter iter = bio->bi_iter;
> +	int idx;
> +
> +	bio_advance_iter(bio, &iter, iter.bi_size);
> +
> +	WARN_ON(!iter.bi_idx && !iter.bi_bvec_done);
> +
> +	if (!iter.bi_bvec_done)
> +		idx = iter.bi_idx - 1;
> +	else	/* in the middle of bvec */
> +		idx = iter.bi_idx;
>
> -	bio_for_each_segment(*bv, bio, iter)
> -		if (bv->bv_len == iter.bi_size)
> -			break;
> +	*bv = bio->bi_io_vec[idx];
> +	if (iter.bi_bvec_done)
> +		bv->bv_len = iter.bi_bvec_done;
>   }
>
>   /*
>

This looks good too.

>
>>
>> However, given that it's a regression bug fix I'm not sure it's the best
>> idea to add logic here.
>
> But the issue is obviously in bio_will_gap(), isn't it?
>
> Simply reverting 52cc6eead9095(block: blk-merge: fast-clone bio when splitting rw bios)
> still might cause performance regression too.

That's correct. I assume that the bio splitting code affects
specific I/O pattern (gappy), however bio_will_gap is also tested
for bio merges (even if the bios won't merge eventually). This means
that each merge check will invoke bio_advance_iter() which is something
I'd like to avoid...