block: relax check on sg gap

BugLink: http://bugs.launchpad.net/bugs/1657539
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@redhat.com>
Reported-by: Dexuan Cui <decui@microsoft.com>
Tested-by: Dexuan Cui <decui@microsoft.com>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jens Axboe <axboe@fb.com>
(cherry picked from linux-next commit 729204ef49ec00b788ce23deb9eb922a5769f55d)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 1ca8e8fd10789d0fff1aa0b21ede64b72075e094..e36a23d0ae3136dcd41f8ff728a8fc9353927544 100644 (file)
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1620,6 +1620,25 @@ 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)
 {
@@ -1629,7 +1648,8 @@ 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;