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

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

Hi,

On Mon, 15 Feb 2016 10:19:49 +0200
Sagi Grimberg <sagig@dev.mellanox.co.il> wrote:

> 
> > +/*
> > + * bio_get_last_bvec() is introduced to get the last bvec of one
> > + * bio for bio_will_gap().
> > + *
> > + * TODO: make it more efficient.
> > + */
> > +static inline void bio_get_last_bvec(struct bio *bio, struct bio_vec *bv)
> > +{
> > +	struct bvec_iter iter;
> > +
> > +	bio_for_each_segment(*bv, bio, iter)
> > +		if (bv->bv_len == iter.bi_size)
> > +			break;
> > +}
> 
> This helper is used for each req/bio once or more. I'd say

No, the helper is only used for the non-splitted BIO, and all
splitted BIO is marked as non-merge.

> it's critical to make it efficient and not settle for
> a quick bail for drivers that don't have a virt_boundary
> like you did in patch #2.

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;
 }
 
 /*


> 
> 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.

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