linux-kernel - [PATCH AUTOSEL 4.14 083/150] block: fix the DISCARD request merge

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Date:   Sat, 16 Nov 2019 10:46:21 -0500
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Jianchao Wang <jianchao.w.wang@...cle.com>,
        Christoph Hellwig <hch@....de>, Ming Lei <ming.lei@...hat.com>,
        Jens Axboe <axboe@...nel.dk>, Sasha Levin <sashal@...nel.org>,
        linux-block@...r.kernel.org
Subject: [PATCH AUTOSEL 4.14 083/150] block: fix the DISCARD request merge

From: Jianchao Wang <jianchao.w.wang@...cle.com>

[ Upstream commit 69840466086d2248898020a08dda52732686c4e6 ]

There are two cases when handle DISCARD merge.
If max_discard_segments == 1, the bios/requests need to be contiguous
to merge. If max_discard_segments > 1, it takes every bio as a range
and different range needn't to be contiguous.

But now, attempt_merge screws this up. It always consider contiguity
for DISCARD for the case max_discard_segments > 1 and cannot merge
contiguous DISCARD for the case max_discard_segments == 1, because
rq_attempt_discard_merge always returns false in this case.
This patch fixes both of the two cases above.

Reviewed-by: Christoph Hellwig <hch@....de>
Reviewed-by: Ming Lei <ming.lei@...hat.com>
Signed-off-by: Jianchao Wang <jianchao.w.wang@...cle.com>
Signed-off-by: Jens Axboe <axboe@...nel.dk>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 block/blk-merge.c | 46 ++++++++++++++++++++++++++++++++++++----------
 1 file changed, 36 insertions(+), 10 deletions(-)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 8d60a5bbcef93..94650cdf2924b 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -659,6 +659,31 @@ static void blk_account_io_merge(struct request *req)
 		part_stat_unlock();
 	}
 }
+/*
+ * Two cases of handling DISCARD merge:
+ * If max_discard_segments > 1, the driver takes every bio
+ * as a range and send them to controller together. The ranges
+ * needn't to be contiguous.
+ * Otherwise, the bios/requests will be handled as same as
+ * others which should be contiguous.
+ */
+static inline bool blk_discard_mergable(struct request *req)
+{
+	if (req_op(req) == REQ_OP_DISCARD &&
+	    queue_max_discard_segments(req->q) > 1)
+		return true;
+	return false;
+}
+
+enum elv_merge blk_try_req_merge(struct request *req, struct request *next)
+{
+	if (blk_discard_mergable(req))
+		return ELEVATOR_DISCARD_MERGE;
+	else if (blk_rq_pos(req) + blk_rq_sectors(req) == blk_rq_pos(next))
+		return ELEVATOR_BACK_MERGE;
+
+	return ELEVATOR_NO_MERGE;
+}
 
 /*
  * For non-mq, this has to be called with the request spinlock acquired.
@@ -676,12 +701,6 @@ static struct request *attempt_merge(struct request_queue *q,
 	if (req_op(req) != req_op(next))
 		return NULL;
 
-	/*
-	 * not contiguous
-	 */
-	if (blk_rq_pos(req) + blk_rq_sectors(req) != blk_rq_pos(next))
-		return NULL;
-
 	if (rq_data_dir(req) != rq_data_dir(next)
 	    || req->rq_disk != next->rq_disk
 	    || req_no_special_merge(next))
@@ -705,11 +724,19 @@ static struct request *attempt_merge(struct request_queue *q,
 	 * counts here. Handle DISCARDs separately, as they
 	 * have separate settings.
 	 */
-	if (req_op(req) == REQ_OP_DISCARD) {
+
+	switch (blk_try_req_merge(req, next)) {
+	case ELEVATOR_DISCARD_MERGE:
 		if (!req_attempt_discard_merge(q, req, next))
 			return NULL;
-	} else if (!ll_merge_requests_fn(q, req, next))
+		break;
+	case ELEVATOR_BACK_MERGE:
+		if (!ll_merge_requests_fn(q, req, next))
+			return NULL;
+		break;
+	default:
 		return NULL;
+	}
 
 	/*
 	 * If failfast settings disagree or any of the two is already
@@ -834,8 +861,7 @@ bool blk_rq_merge_ok(struct request *rq, struct bio *bio)
 
 enum elv_merge blk_try_merge(struct request *rq, struct bio *bio)
 {
-	if (req_op(rq) == REQ_OP_DISCARD &&
-	    queue_max_discard_segments(rq->q) > 1)
+	if (blk_discard_mergable(rq))
 		return ELEVATOR_DISCARD_MERGE;
 	else if (blk_rq_pos(rq) + blk_rq_sectors(rq) == bio->bi_iter.bi_sector)
 		return ELEVATOR_BACK_MERGE;
-- 
2.20.1