[PATCH 07/10] block: Add support for segments smaller than the page size

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

Add the necessary checks in the bio splitting code for splitting bios
into segments smaller than the page size.

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-merge.c | 6 ++++--
 block/blk.h       | 3 ++-
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 35a8f75cc45d..7badfbed09fc 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -294,7 +294,8 @@ static struct bio *bio_split_rw(struct bio *bio, const struct queue_limits *lim,
 		if (nsegs < lim->max_segments &&
 		    bytes + bv.bv_len <= max_bytes &&
 		    bv.bv_offset + bv.bv_len <= PAGE_SIZE) {
-			nsegs++;
+			/* single-page bvec optimization */
+			nsegs += blk_segments(lim, bv.bv_len);
 			bytes += bv.bv_len;
 		} else {
 			if (bvec_split_segs(lim, &bv, &nsegs, &bytes,
@@ -531,7 +532,8 @@ static int __blk_bios_map_sg(struct request_queue *q, struct bio *bio,
 			    __blk_segment_map_sg_merge(q, &bvec, &bvprv, sg))
 				goto next_bvec;
 
-			if (bvec.bv_offset + bvec.bv_len <= PAGE_SIZE)
+			if (bvec.bv_offset + bvec.bv_len <= PAGE_SIZE &&
+			    bvec.bv_len <= q->limits.max_segment_size)
 				nsegs += __blk_bvec_map_sg(bvec, sglist, sg);
 			else
 				nsegs += blk_bvec_map_sg(q, &bvec, sglist, sg);
diff --git a/block/blk.h b/block/blk.h
index 342ba7d86e87..04328211ae3b 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -312,7 +312,7 @@ static inline bool bio_may_exceed_limits(struct bio *bio,
 	}
 
 	/*
-	 * All drivers must accept single-segments bios that are <= PAGE_SIZE.
+	 * Check whether bio splitting should be performed.
 	 * This is a quick and dirty check that relies on the fact that
 	 * bi_io_vec[0] is always valid if a bio has data.  The check might
 	 * lead to occasional false negatives when bios are cloned, but compared
@@ -320,6 +320,7 @@ static inline bool bio_may_exceed_limits(struct bio *bio,
 	 * doesn't matter anyway.
 	 */
 	return lim->chunk_sectors || bio->bi_vcnt != 1 ||
+		bio->bi_io_vec->bv_len > lim->max_segment_size ||
 		bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset > PAGE_SIZE;
 }