[PATCH RFC] block: fix bio merge checks when virt_boundary is set

[Vitaly Kuznetsov]

Hyper-V storage driver, which switched to using virt_boundary some time
ago, experiences significant slowdown on non-page-aligned IO. E.g.

With virt_boundary set:
 # time mkfs.ntfs -Q -s 512 /dev/sdc1
 ...
 real	0m9.406s
 user	0m0.014s
 sys	0m0.672s

Without virt_boundary set (unsafe):
 # time mkfs.ntfs -Q -s 512 /dev/sdc1
 ...
 real	0m6.657s
 user	0m0.012s
 sys	0m6.423s

The reason of the slowdown is the fact that bios don't get merged and we
end up sending many short requests to the host. My investigation led me to
the following code (__bvec_gap_to_prev()):

    return offset ||
           ((bprv->bv_offset + bprv->bv_len) & queue_virt_boundary(q));

Here is an example: we have two bio_vec with the following content:
    bprv.bv_offset = 512
    bprv.bv_len = 512

    bnxt.bv_offset = 1024
    bnxt.bv_len = 512

    bprv.bv_page == bnxt.bv_page
    virt_boundary is set to PAGE_SIZE-1

The above mentioned code will report that a gap will appear if we merge
these two (as offset = 1024) but this doesn't look sane. On top of that,
we have the following optimization in bio_add_pc_page():

    if (page == prev->bv_page &&
        offset == prev->bv_offset + prev->bv_len) {
            prev->bv_len += len;
            bio->bi_iter.bi_size += len;
            goto done;
        }

But we don't have such check in other places, which check virt_boundary.
Modify the check in __bvec_gap_to_prev() to the following:
1) Report no gap in case bnxt->bv_offset == bprv->bv_offset + bprv->bv_len
   when bprv.bv_page == bnxt.bv_page.
2) Continue reporting no gap in (bprv->bv_offset + bprv->bv_len) &
   queue_virt_boundary(q) case.

Reported-by: John R. Kozee II <jkozee@xxxxxxxxxxxxxxxxx>
Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
---
- The condition I'm changing was there since SG_GAPS so I may be missing
  something important, thus RFC.
---
 block/bio-integrity.c  |  7 +++++--
 block/bio.c            |  4 +++-
 block/blk-merge.c      |  2 +-
 include/linux/blkdev.h | 17 +++++++++--------
 4 files changed, 18 insertions(+), 12 deletions(-)

diff --git a/block/bio-integrity.c b/block/bio-integrity.c
index 711e4d8d..f8560da 100644
--- a/block/bio-integrity.c
+++ b/block/bio-integrity.c
@@ -136,7 +136,7 @@ int bio_integrity_add_page(struct bio *bio, struct page *page,
 			   unsigned int len, unsigned int offset)
 {
 	struct bio_integrity_payload *bip = bio_integrity(bio);
-	struct bio_vec *iv;
+	struct bio_vec *iv, bv;
 
 	if (bip->bip_vcnt >= bip->bip_max_vcnt) {
 		printk(KERN_ERR "%s: bip_vec full\n", __func__);
@@ -144,10 +144,13 @@ int bio_integrity_add_page(struct bio *bio, struct page *page,
 	}
 
 	iv = bip->bip_vec + bip->bip_vcnt;
+	bv.bv_page = page;
+	bv.bv_len = len;
+	bv.bv_offset = offset;
 
 	if (bip->bip_vcnt &&
 	    bvec_gap_to_prev(bdev_get_queue(bio->bi_bdev),
-			     &bip->bip_vec[bip->bip_vcnt - 1], offset))
+			     &bip->bip_vec[bip->bip_vcnt - 1], &bv))
 		return 0;
 
 	iv->bv_page = page;
diff --git a/block/bio.c b/block/bio.c
index cf75915..1583581 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -730,6 +730,8 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
 	 */
 	if (bio->bi_vcnt > 0) {
 		struct bio_vec *prev = &bio->bi_io_vec[bio->bi_vcnt - 1];
+		struct bio_vec bv = {.bv_page = page, .bv_len = len,
+				     .bv_offset = offset};
 
 		if (page == prev->bv_page &&
 		    offset == prev->bv_offset + prev->bv_len) {
@@ -742,7 +744,7 @@ int bio_add_pc_page(struct request_queue *q, struct bio *bio, struct page
 		 * If the queue doesn't support SG gaps and adding this
 		 * offset would create a gap, disallow it.
 		 */
-		if (bvec_gap_to_prev(q, prev, offset))
+		if (bvec_gap_to_prev(q, prev, &bv))
 			return 0;
 	}
 
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 2613531..8c6c3e2 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -100,7 +100,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
 		 * If the queue doesn't support SG gaps and adding this
 		 * offset would create a gap, disallow it.
 		 */
-		if (bvprvp && bvec_gap_to_prev(q, bvprvp, bv.bv_offset))
+		if (bvprvp && bvec_gap_to_prev(q, bvprvp, &bv))
 			goto split;
 
 		if (sectors + (bv.bv_len >> 9) > max_sectors) {
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 413c84f..b4fa29d 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -1373,10 +1373,11 @@ static inline void put_dev_sector(Sector p)
 }
 
 static inline bool __bvec_gap_to_prev(struct request_queue *q,
-				struct bio_vec *bprv, unsigned int offset)
+				struct bio_vec *bprv, struct bio_vec *bnxt)
 {
-	return offset ||
-		((bprv->bv_offset + bprv->bv_len) & queue_virt_boundary(q));
+	if (bprv->bv_page == bnxt->bv_page)
+		return bnxt->bv_offset != bprv->bv_offset + bprv->bv_len;
+	return (bprv->bv_offset + bprv->bv_len) & queue_virt_boundary(q);
 }
 
 /*
@@ -1384,11 +1385,11 @@ static inline bool __bvec_gap_to_prev(struct request_queue *q,
  * the SG list. Most drivers don't care about this, but some do.
  */
 static inline bool bvec_gap_to_prev(struct request_queue *q,
-				struct bio_vec *bprv, unsigned int offset)
+				struct bio_vec *bprv, struct bio_vec *bnxt)
 {
 	if (!queue_virt_boundary(q))
 		return false;
-	return __bvec_gap_to_prev(q, bprv, offset);
+	return __bvec_gap_to_prev(q, bprv, bnxt);
 }
 
 static inline bool bio_will_gap(struct request_queue *q, struct bio *prev,
@@ -1400,7 +1401,7 @@ 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);
+		return __bvec_gap_to_prev(q, &pb, &nb);
 	}
 
 	return false;
@@ -1545,7 +1546,7 @@ static inline bool integrity_req_gap_back_merge(struct request *req,
 	struct bio_integrity_payload *bip_next = bio_integrity(next);
 
 	return bvec_gap_to_prev(req->q, &bip->bip_vec[bip->bip_vcnt - 1],
-				bip_next->bip_vec[0].bv_offset);
+				&bip_next->bip_vec[0]);
 }
 
 static inline bool integrity_req_gap_front_merge(struct request *req,
@@ -1555,7 +1556,7 @@ static inline bool integrity_req_gap_front_merge(struct request *req,
 	struct bio_integrity_payload *bip_next = bio_integrity(req->bio);
 
 	return bvec_gap_to_prev(req->q, &bip->bip_vec[bip->bip_vcnt - 1],
-				bip_next->bip_vec[0].bv_offset);
+				&bip_next->bip_vec[0]);
 }
 
 #else /* CONFIG_BLK_DEV_INTEGRITY */
-- 
2.5.0