block: fix the DISCARD request merge

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@lst.de>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 36 insertions, 10 deletions

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 42a46744c11b..6b5ad275ed56 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -714,6 +714,31 @@ 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.
@@ -731,12 +756,6 @@ 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))
@@ -760,11 +779,19 @@ 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
@@ -888,8 +915,7 @@ 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;