[PATCH 06/10] block: Fix the number of segment calculations

[Bart Van Assche <bvanassche@acm.org>]

Since multi-page bvecs are supported there can be multiple segments per
bvec (see also bvec_split_segs()). Hence this patch that calculates the
number of segments per bvec instead of assuming that there is only one
segment per bvec.

Cc: Christoph Hellwig <hch@lst.de>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Keith Busch <kbusch@kernel.org>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
---
 block/blk-map.c | 14 +++++++++++++-
 block/blk-mq.c  |  2 ++
 block/blk.h     |  3 +++
 3 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/block/blk-map.c b/block/blk-map.c
index 4505307d758e..2976a8d68992 100644
--- a/block/blk-map.c
+++ b/block/blk-map.c
@@ -529,6 +529,18 @@ static struct bio *bio_copy_kern(struct request_queue *q, void *data,
 	return ERR_PTR(-ENOMEM);
 }
 
+/* Number of DMA segments required to transfer @bytes data. */
+unsigned int blk_segments(const struct queue_limits *limits, unsigned int bytes)
+{
+	const unsigned int mss = limits->max_segment_size;
+
+	if (bytes <= mss)
+		return 1;
+	if (is_power_of_2(mss))
+		return round_up(bytes, mss) >> ilog2(mss);
+	return (bytes + mss - 1) / mss;
+}
+
 /*
  * Append a bio to a passthrough request.  Only works if the bio can be merged
  * into the request based on the driver constraints.
@@ -540,7 +552,7 @@ int blk_rq_append_bio(struct request *rq, struct bio *bio)
 	unsigned int nr_segs = 0;
 
 	bio_for_each_bvec(bv, bio, iter)
-		nr_segs++;
+		nr_segs += blk_segments(&rq->q->limits, bv.bv_len);
 
 	if (!rq->bio) {
 		blk_rq_bio_prep(rq, bio, nr_segs);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8070b6c10e8d..82ae8099afdd 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2883,6 +2883,8 @@ void blk_mq_submit_bio(struct bio *bio)
 	bio = blk_queue_bounce(bio, q);
 	if (bio_may_exceed_limits(bio, &q->limits))
 		bio = __bio_split_to_limits(bio, &q->limits, &nr_segs);
+	else if (bio->bi_vcnt == 1)
+		nr_segs = blk_segments(&q->limits, bio->bi_io_vec[0].bv_len);
 
 	if (!bio_integrity_prep(bio))
 		return;
diff --git a/block/blk.h b/block/blk.h
index 7f9e089ab1f7..342ba7d86e87 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -81,6 +81,9 @@ struct bio_vec *bvec_alloc(mempool_t *pool, unsigned short *nr_vecs,
 		gfp_t gfp_mask);
 void bvec_free(mempool_t *pool, struct bio_vec *bv, unsigned short nr_vecs);
 
+unsigned int blk_segments(const struct queue_limits *limits,
+			  unsigned int bytes);
+
 static inline bool biovec_phys_mergeable(struct request_queue *q,
 		struct bio_vec *vec1, struct bio_vec *vec2)
 {