[SRU][Xenial][Yakkety][PATCH 1/1] block: relax check on sg gap

[Joseph Salisbury]

From: Ming Lei <ming.lei at canonical.com>

BugLink: http://bugs.launchpad.net/bugs/1682215

If the last bvec of the 1st bio and the 1st bvec of the next
bio are physically contigious, and the latter can be merged
to last segment of the 1st bio, we should think they don't
violate sg gap(or virt boundary) limit.

Both Vitaly and Dexuan reported lots of unmergeable small bios
are observed when running mkfs on Hyper-V virtual storage, and
performance becomes quite low. This patch fixes that performance
issue.

The same issue should exist on NVMe, since it sets virt boundary too.

Reported-by: Vitaly Kuznetsov <vkuznets at redhat.com>
Reported-by: Dexuan Cui <decui at microsoft.com>
Tested-by: Dexuan Cui <decui at microsoft.com>
Cc: Keith Busch <keith.busch at intel.com>
Signed-off-by: Ming Lei <ming.lei at canonical.com>
Signed-off-by: Jens Axboe <axboe at fb.com>
(cherry picked from commit 729204ef49ec00b788ce23deb9eb922a5769f55d)
Signed-off-by: Joseph Salisbury <joseph.salisbury at canonical.com>
---
 include/linux/blkdev.h | 22 +++++++++++++++++++++-
 1 file changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index f28a95a..a0b6aae 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1387,6 +1387,25 @@ static inline bool bvec_gap_to_prev(struct request_queue *q,
 	return __bvec_gap_to_prev(q, bprv, offset);
 }
 
+/*
+ * Check if the two bvecs from two bios can be merged to one segment.
+ * If yes, no need to check gap between the two bios since the 1st bio
+ * and the 1st bvec in the 2nd bio can be handled in one segment.
+ */
+static inline bool bios_segs_mergeable(struct request_queue *q,
+		struct bio *prev, struct bio_vec *prev_last_bv,
+		struct bio_vec *next_first_bv)
+{
+	if (!BIOVEC_PHYS_MERGEABLE(prev_last_bv, next_first_bv))
+		return false;
+	if (!BIOVEC_SEG_BOUNDARY(q, prev_last_bv, next_first_bv))
+		return false;
+	if (prev->bi_seg_back_size + next_first_bv->bv_len >
+			queue_max_segment_size(q))
+		return false;
+	return true;
+}
+
 static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
 			 struct bio *next)
 {
@@ -1396,7 +1415,8 @@ static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
 		bio_get_last_bvec(prev, &pb);
 		bio_get_first_bvec(next, &nb);
 
-		return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
+		if (!bios_segs_mergeable(q, prev, &pb, &nb))
+			return __bvec_gap_to_prev(q, &pb, nb.bv_offset);
 	}
 
 	return false;
-- 
2.7.4