dm: requeue from blk-mq dm_mq_queue_rq() using BLK_MQ_RQ_QUEUE_BUSY

Use BLK_MQ_RQ_QUEUE_BUSY to requeue a blk-mq request directly from the
DM blk-mq device's .queue_rq.  This cleans up the previous convoluted
handling of request requeueing that would return BLK_MQ_RQ_QUEUE_OK
(even though it wasn't) and then run blk_mq_requeue_request() followed
by blk_mq_kick_requeue_list().

Also, document that DM blk-mq ontop of old request_fn devices cannot
fail in clone_rq() since the clone request is preallocated as part of
the pdu.

Reported-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

1 files changed, 6 insertions, 4 deletions

diff --git a/drivers/md/dm.c b/drivers/md/dm.c
index 1c62ed8d09f4..1badfb250a18 100644
--- a/drivers/md/dm.c
+++ b/drivers/md/dm.c
@@ -2754,13 +2754,15 @@ static int dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
         if (dm_table_get_type(map) == DM_TYPE_REQUEST_BASED) {
                 /* clone request is allocated at the end of the pdu */
                 tio->clone = (void *)blk_mq_rq_to_pdu(rq) + sizeof(struct dm_rq_target_io);
-                if (!clone_rq(rq, md, tio, GFP_ATOMIC))
-                        return BLK_MQ_RQ_QUEUE_BUSY;
+                (void) clone_rq(rq, md, tio, GFP_ATOMIC);
                 queue_kthread_work(&md->kworker, &tio->work);
         } else {
                 /* Direct call is fine since .queue_rq allows allocations */
-                if (map_request(tio, rq, md) == DM_MAPIO_REQUEUE)
-                        dm_requeue_unmapped_original_request(md, rq);
+                if (map_request(tio, rq, md) == DM_MAPIO_REQUEUE) {
+                        /* Undo dm_start_request() before requeuing */
+                        rq_completed(md, rq_data_dir(rq), false);
+                        return BLK_MQ_RQ_QUEUE_BUSY;
+                }
         }
 
         return BLK_MQ_RQ_QUEUE_OK;