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

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