blk-mq: make plug work for mutiple disks and queues

Last patch makes plug work for multiple queue case. However it only
works for single disk case, because it assumes only one request in the
plug list. If a task is accessing multiple disks, eg MD/DM, the
assumption is wrong. Let blk_attempt_plug_merge() record request from
the same queue.

V2: use NULL parameter in !mq case. Fix a bug. Add comments in
blk_attempt_plug_merge to make it less (hopefully) confusion.

Cc: Jens Axboe <axboe@fb.com>
Cc: Christoph Hellwig <hch@lst.de>
Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: Jens Axboe <axboe@fb.com>

1 files changed, 9 insertions, 5 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 902c2eb9a0e7..31df47443699 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1269,6 +1269,7 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
         struct request *rq;
         unsigned int request_count = 0;
         struct blk_plug *plug;
+        struct request *same_queue_rq = NULL;
 
         blk_queue_bounce(q, &bio);
 
@@ -1278,7 +1279,7 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
         }
 
         if (!is_flush_fua && !blk_queue_nomerges(q) &&
-            blk_attempt_plug_merge(q, bio, &request_count))
+            blk_attempt_plug_merge(q, bio, &request_count, &same_queue_rq))
                 return;
 
         rq = blk_mq_map_request(q, bio, &data);
@@ -1309,9 +1310,12 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
                  * issued. So the plug list will have one request at most
                  */
                 if (plug) {
-                        if (!list_empty(&plug->mq_list)) {
-                                old_rq = list_first_entry(&plug->mq_list,
-                                        struct request, queuelist);
+                        /*
+                         * The plug list might get flushed before this. If that
+                         * happens, same_queue_rq is invalid and plug list is empty
+                         **/
+                        if (same_queue_rq && !list_empty(&plug->mq_list)) {
+                                old_rq = same_queue_rq;
                                 list_del_init(&old_rq->queuelist);
                         }
                         list_add_tail(&rq->queuelist, &plug->mq_list);
@@ -1360,7 +1364,7 @@ static void blk_sq_make_request(struct request_queue *q, struct bio *bio)
         }
 
         if (!is_flush_fua && !blk_queue_nomerges(q) &&
-            blk_attempt_plug_merge(q, bio, &request_count))
+            blk_attempt_plug_merge(q, bio, &request_count, NULL))
                 return;
 
         rq = blk_mq_map_request(q, bio, &data);