dm: add full blk-mq support to request-based DM

Commit e5863d9a ("dm: allocate requests in target when stacking on
blk-mq devices") served as the first step toward fully utilizing blk-mq
in request-based DM -- it enabled stacking an old-style (request_fn)
request_queue ontop of the underlying blk-mq device(s).  That first step
didn't improve performance of DM multipath ontop of fast blk-mq devices
(e.g. NVMe) because the top-level old-style request_queue was severely
limited by the queue_lock.

The second step offered here enables stacking a blk-mq request_queue
ontop of the underlying blk-mq device(s).  This unlocks significant
performance gains on fast blk-mq devices, Keith Busch tested on his NVMe
testbed and offered this really positive news:

 "Just providing a performance update. All my fio tests are getting
  roughly equal performance whether accessed through the raw block
  device or the multipath device mapper (~470k IOPS). I could only push
  ~20% of the raw iops through dm before this conversion, so this latest
  tree is looking really solid from a performance standpoint."
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Tested-by: Keith Busch <keith.busch@intel.com>

drivers/md/dm-mpath.c

@@ -1703,7 +1703,7 @@ static int multipath_busy(struct dm_target *ti)
  *---------------------------------------------------------------*/
 static struct target_type multipath_target = {
 	.name = "multipath",
-	.version = {1, 8, 0},
+	.version = {1, 9, 0},
 	.module = THIS_MODULE,
 	.ctr = multipath_ctr,
 	.dtr = multipath_dtr,

drivers/md/dm-table.c

@@ -18,6 +18,7 @@
 #include <linux/mutex.h>
 #include <linux/delay.h>
 #include <linux/atomic.h>
+#include <linux/blk-mq.h>
 
 #define DM_MSG_PREFIX "table"
 
@@ -1695,10 +1696,14 @@ void dm_table_run_md_queue_async(struct dm_table *t)
 	md = dm_table_get_md(t);
 	queue = dm_get_md_queue(md);
 	if (queue) {
+		if (queue->mq_ops)
+			blk_mq_run_hw_queues(queue, true);
+		else {
 			spin_lock_irqsave(queue->queue_lock, flags);
 			blk_run_queue_async(queue);
 			spin_unlock_irqrestore(queue->queue_lock, flags);
 		}
+	}
 }
 EXPORT_SYMBOL(dm_table_run_md_queue_async);
 

include/uapi/linux/dm-ioctl.h

@@ -267,9 +267,9 @@ enum {
 #define DM_DEV_SET_GEOMETRY	_IOWR(DM_IOCTL, DM_DEV_SET_GEOMETRY_CMD, struct dm_ioctl)
 
 #define DM_VERSION_MAJOR	4
-#define DM_VERSION_MINOR	30
+#define DM_VERSION_MINOR	31
 #define DM_VERSION_PATCHLEVEL	0
-#define DM_VERSION_EXTRA	"-ioctl (2014-12-22)"
+#define DM_VERSION_EXTRA	"-ioctl (2015-3-12)"
 
 /* Status bits */
 #define DM_READONLY_FLAG	(1 << 0) /* In/Out */