[PATCH] block: Merge discard requests as a special case

[Keith Busch]

Discard requests operate under different constraints than other operations
and have different rules for merging. This patch will treat such requests
as a special case, using the same criteria and segment accounting used
for merging a discard bio into a reqseut.

Signed-off-by: Keith Busch <keith.bu...@intel.com>
---
 block/blk-merge.c | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/block/blk-merge.c b/block/blk-merge.c
index 8452fc7164cc..37186fb02cf1 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -550,6 +550,25 @@ static bool req_no_special_merge(struct request *req)
        return !q->mq_ops && req->special;
 }
 
+static int req_attempt_discard_merge(struct request_queue *q, struct request 
*req,
+               struct request *next)
+{
+       unsigned short segments = blk_rq_nr_discard_segments(req);
+
+       if (segments >= queue_max_discard_segments(q))
+               goto no_merge;
+       if (blk_rq_sectors(req) + bio_sectors(next->bio) >
+           blk_rq_get_max_sectors(req, blk_rq_pos(req)))
+               goto no_merge;
+
+       req->nr_phys_segments = segments + blk_rq_nr_discard_segments(next);
+       return 1;
+
+no_merge:
+       req_set_nomerge(q, req);
+       return 0;
+}
+
 static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
                                struct request *next)
 {
@@ -564,6 +583,13 @@ static int ll_merge_requests_fn(struct request_queue *q, 
struct request *req,
        if (req_no_special_merge(req) || req_no_special_merge(next))
                return 0;
 
+       /*
+        * Merging discard requests use different constraints than other
+        * operations.
+        */
+       if (req_op(req) == REQ_OP_DISCARD)
+               return req_attempt_discard_merge(q, req, next);
+
        if (req_gap_back_merge(req, next->bio))
                return 0;
 
-- 
2.14.3