Messages in this thread | | | Subject | Re: [PATCH 1/4] block: bio: introduce helpers to get the 1st and last bvec | From | Sagi Grimberg <> | Date | Mon, 15 Feb 2016 22:06:47 +0200 |
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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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