--- /dev/null
+From 5a8d75a1b8c99bdc926ba69b7b7dbe4fae81a5af Mon Sep 17 00:00:00 2001
+From: Ming Lei <ming.lei@redhat.com>
+Date: Fri, 14 Apr 2017 13:58:29 -0600
+Subject: [PATCH] block: fix bio_will_gap() for first bvec with offset
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+Commit 729204ef49ec("block: relax check on sg gap") allows us to merge
+bios, if both are physically contiguous. This change can merge a huge
+number of small bios, through mkfs for example, mkfs.ntfs running time
+can be decreased to ~1/10.
+
+But if one rq starts with a non-aligned buffer (the 1st bvec's bv_offset
+is non-zero) and if we allow the merge, it is quite difficult to respect
+sg gap limit, especially the max segment size, or we risk having an
+unaligned virtual boundary. This patch tries to avoid the issue by
+disallowing a merge, if the req starts with an unaligned buffer.
+
+Also add comments to explain why the merged segment can't end in
+unaligned virt boundary.
+
+Fixes: 729204ef49ec ("block: relax check on sg gap")
+Tested-by: Johannes Thumshirn <jthumshirn@suse.de>
+Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
+Signed-off-by: Ming Lei <ming.lei@redhat.com>
+
+Rewrote parts of the commit message and comments.
+
+Signed-off-by: Jens Axboe <axboe@fb.com>
+Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>
+---
+ include/linux/blkdev.h | 32 ++++++++++++++++++++++++++++----
+ 1 file changed, 28 insertions(+), 4 deletions(-)
+
+diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
+index 7548f332121a..01a696b0a4d3 100644
+--- a/include/linux/blkdev.h
++++ b/include/linux/blkdev.h
+@@ -1672,12 +1672,36 @@ static inline bool bios_segs_mergeable(struct request_queue *q,
+ return true;
+ }
+
+-static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
+- struct bio *next)
++static inline bool bio_will_gap(struct request_queue *q,
++ struct request *prev_rq,
++ struct bio *prev,
++ struct bio *next)
+ {
+ if (bio_has_data(prev) && queue_virt_boundary(q)) {
+ struct bio_vec pb, nb;
+
++ /*
++ * don't merge if the 1st bio starts with non-zero
++ * offset, otherwise it is quite difficult to respect
++ * sg gap limit. We work hard to merge a huge number of small
++ * single bios in case of mkfs.
++ */
++ if (prev_rq)
++ bio_get_first_bvec(prev_rq->bio, &pb);
++ else
++ bio_get_first_bvec(prev, &pb);
++ if (pb.bv_offset)
++ return true;
++
++ /*
++ * We don't need to worry about the situation that the
++ * merged segment ends in unaligned virt boundary:
++ *
++ * - if 'pb' ends aligned, the merged segment ends aligned
++ * - if 'pb' ends unaligned, the next bio must include
++ * one single bvec of 'nb', otherwise the 'nb' can't
++ * merge with 'pb'
++ */
+ bio_get_last_bvec(prev, &pb);
+ bio_get_first_bvec(next, &nb);
+
+@@ -1690,12 +1714,12 @@ static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
+
+ static inline bool req_gap_back_merge(struct request *req, struct bio *bio)
+ {
+- return bio_will_gap(req->q, req->biotail, bio);
++ return bio_will_gap(req->q, req, req->biotail, bio);
+ }
+
+ static inline bool req_gap_front_merge(struct request *req, struct bio *bio)
+ {
+- return bio_will_gap(req->q, bio, req->bio);
++ return bio_will_gap(req->q, NULL, bio, req->bio);
+ }
+
+ int kblockd_schedule_work(struct work_struct *work);
+--
+2.11.0
+