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

Commit e5863d9ad ("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 <[e-mail]>
Tested-by: Keith Busch <[e-mail]>

drivers/md/dm-mpath.c

···
  *---------------------------------------------------------------*/
 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

···
 #include <linux/mutex.h>
 #include <linux/delay.h>
 #include <linux/atomic.h>
+#include <linux/blk-mq.h>
 
 #define DM_MSG_PREFIX "table"
 
···
 	md = dm_table_get_md(t);
 	queue = dm_get_md_queue(md);
 	if (queue) {
-		spin_lock_irqsave(queue->queue_lock, flags);
-		blk_run_queue_async(queue);
-		spin_unlock_irqrestore(queue->queue_lock, flags);
+		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);

drivers/md/dm.c

···
 #include <linux/kthread.h>
 #include <linux/ktime.h>
 #include <linux/elevator.h> /* for rq_end_sector() */
+#include <linux/blk-mq.h>
 
 #include <trace/events/block.h>
 
···
 	int last_rq_rw;
 	sector_t last_rq_pos;
 	ktime_t last_rq_start_time;
+
+	/* for blk-mq request-based DM support */
+	struct blk_mq_tag_set tag_set;
 };
 
 /*
···
 	blk_update_request(tio->orig, 0, nr_bytes);
 }
 
+static struct dm_rq_target_io *tio_from_request(struct request *rq)
+{
+	return (rq->q->mq_ops ? blk_mq_rq_to_pdu(rq) : rq->special);
+}
+
 /*
  * Don't touch any member of the md after calling this function because
  * the md may be freed in dm_put() at the end of this function.
···
 	 * queue lock again.
 	 */
 	if (run_queue) {
-		if (!nr_requests_pending ||
-		    (nr_requests_pending >= md->queue->nr_congestion_on))
+		if (md->queue->mq_ops)
+			blk_mq_run_hw_queues(md->queue, true);
+		else if (!nr_requests_pending ||
+			 (nr_requests_pending >= md->queue->nr_congestion_on))
 			blk_run_queue_async(md->queue);
 	}
 
···
 static void free_rq_clone(struct request *clone)
 {
 	struct dm_rq_target_io *tio = clone->end_io_data;
+	struct mapped_device *md = tio->md;
 
 	blk_rq_unprep_clone(clone);
+
 	if (clone->q && clone->q->mq_ops)
 		tio->ti->type->release_clone_rq(clone);
 	else
-		free_clone_request(tio->md, clone);
-	free_rq_tio(tio);
+		free_clone_request(md, clone);
+
+	if (!md->queue->mq_ops)
+		free_rq_tio(tio);
 }
 
 /*
···
 	}
 
 	free_rq_clone(clone);
-	blk_end_request_all(rq, error);
+	if (!rq->q->mq_ops)
+		blk_end_request_all(rq, error);
+	else
+		blk_mq_end_request(rq, error);
 	rq_completed(md, rw, true);
 }
 
 static void dm_unprep_request(struct request *rq)
 {
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_rq_target_io *tio = tio_from_request(rq);
 	struct request *clone = tio->clone;
 
-	rq->special = NULL;
-	rq->cmd_flags &= ~REQ_DONTPREP;
+	if (!rq->q->mq_ops) {
+		rq->special = NULL;
+		rq->cmd_flags &= ~REQ_DONTPREP;
+	}
 
 	if (clone)
 		free_rq_clone(clone);
···
 /*
  * Requeue the original request of a clone.
  */
-static void dm_requeue_unmapped_original_request(struct mapped_device *md,
-						 struct request *rq)
+static void old_requeue_request(struct request *rq)
 {
-	int rw = rq_data_dir(rq);
 	struct request_queue *q = rq->q;
 	unsigned long flags;
-
-	dm_unprep_request(rq);
 
 	spin_lock_irqsave(q->queue_lock, flags);
 	blk_requeue_request(q, rq);
 	spin_unlock_irqrestore(q->queue_lock, flags);
+}
+
+static void dm_requeue_unmapped_original_request(struct mapped_device *md,
+						 struct request *rq)
+{
+	int rw = rq_data_dir(rq);
+
+	dm_unprep_request(rq);
+
+	if (!rq->q->mq_ops)
+		old_requeue_request(rq);
+	else {
+		blk_mq_requeue_request(rq);
+		blk_mq_kick_requeue_list(rq->q);
+	}
 
 	rq_completed(md, rw, false);
 }
···
 	dm_requeue_unmapped_original_request(tio->md, tio->orig);
 }
 
-static void __stop_queue(struct request_queue *q)
+static void old_stop_queue(struct request_queue *q)
 {
+	unsigned long flags;
+
+	if (blk_queue_stopped(q))
+		return;
+
+	spin_lock_irqsave(q->queue_lock, flags);
 	blk_stop_queue(q);
+	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
 static void stop_queue(struct request_queue *q)
 {
+	if (!q->mq_ops)
+		old_stop_queue(q);
+	else
+		blk_mq_stop_hw_queues(q);
+}
+
+static void old_start_queue(struct request_queue *q)
+{
 	unsigned long flags;
 
 	spin_lock_irqsave(q->queue_lock, flags);
-	__stop_queue(q);
-	spin_unlock_irqrestore(q->queue_lock, flags);
-}
-
-static void __start_queue(struct request_queue *q)
-{
 	if (blk_queue_stopped(q))
 		blk_start_queue(q);
+	spin_unlock_irqrestore(q->queue_lock, flags);
 }
 
 static void start_queue(struct request_queue *q)
 {
-	unsigned long flags;
-
-	spin_lock_irqsave(q->queue_lock, flags);
-	__start_queue(q);
-	spin_unlock_irqrestore(q->queue_lock, flags);
+	if (!q->mq_ops)
+		old_start_queue(q);
+	else
+		blk_mq_start_stopped_hw_queues(q, true);
 }
 
 static void dm_done(struct request *clone, int error, bool mapped)
···
 static void dm_softirq_done(struct request *rq)
 {
 	bool mapped = true;
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_rq_target_io *tio = tio_from_request(rq);
 	struct request *clone = tio->clone;
+	int rw;
 
 	if (!clone) {
-		blk_end_request_all(rq, tio->error);
-		rq_completed(tio->md, rq_data_dir(rq), false);
-		free_rq_tio(tio);
+		rw = rq_data_dir(rq);
+		if (!rq->q->mq_ops) {
+			blk_end_request_all(rq, tio->error);
+			rq_completed(tio->md, rw, false);
+			free_rq_tio(tio);
+		} else {
+			blk_mq_end_request(rq, tio->error);
+			rq_completed(tio->md, rw, false);
+		}
 		return;
 	}
 
···
  */
 static void dm_complete_request(struct request *rq, int error)
 {
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_rq_target_io *tio = tio_from_request(rq);
 
 	tio->error = error;
 	blk_complete_request(rq);
···
 }
 
 /*
- * Called with the clone's queue lock held
+ * Called with the clone's queue lock held (for non-blk-mq)
  */
 static void end_clone_request(struct request *clone, int error)
 {
···
 
 static void map_tio_request(struct kthread_work *work);
 
+static void init_tio(struct dm_rq_target_io *tio, struct request *rq,
+		     struct mapped_device *md)
+{
+	tio->md = md;
+	tio->ti = NULL;
+	tio->clone = NULL;
+	tio->orig = rq;
+	tio->error = 0;
+	memset(&tio->info, 0, sizeof(tio->info));
+	init_kthread_work(&tio->work, map_tio_request);
+}
+
 static struct dm_rq_target_io *prep_tio(struct request *rq,
 					struct mapped_device *md, gfp_t gfp_mask)
 {
···
 	if (!tio)
 		return NULL;
 
-	tio->md = md;
-	tio->ti = NULL;
-	tio->clone = NULL;
-	tio->orig = rq;
-	tio->error = 0;
-	memset(&tio->info, 0, sizeof(tio->info));
-	init_kthread_work(&tio->work, map_tio_request);
+	init_tio(tio, rq, md);
 
 	table = dm_get_live_table(md, &srcu_idx);
 	if (!dm_table_mq_request_based(table)) {
···
  * DM_MAPIO_REQUEUE : the original request needs to be requeued
  * < 0              : the request was completed due to failure
  */
-static int map_request(struct dm_target *ti, struct request *rq,
+static int map_request(struct dm_rq_target_io *tio, struct request *rq,
 		       struct mapped_device *md)
 {
 	int r;
-	struct dm_rq_target_io *tio = rq->special;
+	struct dm_target *ti = tio->ti;
 	struct request *clone = NULL;
 
 	if (tio->clone) {
···
 		}
 		if (IS_ERR(clone))
 			return DM_MAPIO_REQUEUE;
-		if (setup_clone(clone, rq, tio, GFP_KERNEL)) {
+		if (setup_clone(clone, rq, tio, GFP_NOIO)) {
 			/* -ENOMEM */
 			ti->type->release_clone_rq(clone);
 			return DM_MAPIO_REQUEUE;
···
 	struct request *rq = tio->orig;
 	struct mapped_device *md = tio->md;
 
-	if (map_request(tio->ti, rq, md) == DM_MAPIO_REQUEUE)
+	if (map_request(tio, rq, md) == DM_MAPIO_REQUEUE)
 		dm_requeue_unmapped_original_request(md, rq);
 }
 
 static void dm_start_request(struct mapped_device *md, struct request *orig)
 {
-	blk_start_request(orig);
+	if (!orig->q->mq_ops)
+		blk_start_request(orig);
+	else
+		blk_mq_start_request(orig);
 	atomic_inc(&md->pending[rq_data_dir(orig)]);
 
 	if (md->seq_rq_merge_deadline_usecs) {
···
 
 		dm_start_request(md, rq);
 
-		tio = rq->special;
+		tio = tio_from_request(rq);
 		/* Establish tio->ti before queuing work (map_tio_request) */
 		tio->ti = ti;
 		queue_kthread_work(&md->kworker, &tio->work);
···
 {
 	/*
 	 * Request-based dm devices cannot be stacked on top of bio-based dm
-	 * devices.  The type of this dm device has not been decided yet.
+	 * devices.  The type of this dm device may not have been decided yet.
 	 * The type is decided at the first table loading time.
 	 * To prevent problematic device stacking, clear the queue flag
 	 * for request stacking support until then.
···
 	 * This queue is new, so no concurrency on the queue_flags.
 	 */
 	queue_flag_clear_unlocked(QUEUE_FLAG_STACKABLE, md->queue);
+}
 
+static void dm_init_old_md_queue(struct mapped_device *md)
+{
+	dm_init_md_queue(md);
+
+	/*
+	 * Initialize aspects of queue that aren't relevant for blk-mq
+	 */
 	md->queue->queuedata = md;
 	md->queue->backing_dev_info.congested_fn = dm_any_congested;
 	md->queue->backing_dev_info.congested_data = md;
···
 static void free_dev(struct mapped_device *md)
 {
 	int minor = MINOR(disk_devt(md->disk));
+	bool using_blk_mq = !!md->queue->mq_ops;
 
 	unlock_fs(md);
 	destroy_workqueue(md->wq);
···
 	del_gendisk(md->disk);
 	put_disk(md->disk);
 	blk_cleanup_queue(md->queue);
+	if (using_blk_mq)
+		blk_mq_free_tag_set(&md->tag_set);
 	bdput(md->bdev);
 	free_minor(minor);
 
···
 	 * This must be done before setting the queue restrictions,
 	 * because request-based dm may be run just after the setting.
 	 */
-	if (dm_table_request_based(t) && !blk_queue_stopped(q))
+	if (dm_table_request_based(t))
 		stop_queue(q);
 
 	__bind_mempools(md, t);
···
 	return md->type;
 }
 
-static bool dm_md_type_request_based(struct mapped_device *md)
-{
-	unsigned table_type = dm_get_md_type(md);
-
-	return (table_type == DM_TYPE_REQUEST_BASED ||
-		table_type == DM_TYPE_MQ_REQUEST_BASED);
-}
-
 struct target_type *dm_get_immutable_target_type(struct mapped_device *md)
 {
 	return md->immutable_target_type;
···
 }
 EXPORT_SYMBOL_GPL(dm_get_queue_limits);
 
+static void init_rq_based_worker_thread(struct mapped_device *md)
+{
+	/* Initialize the request-based DM worker thread */
+	init_kthread_worker(&md->kworker);
+	md->kworker_task = kthread_run(kthread_worker_fn, &md->kworker,
+				       "kdmwork-%s", dm_device_name(md));
+}
+
 /*
  * Fully initialize a request-based queue (->elevator, ->request_fn, etc).
  */
···
 	struct request_queue *q = NULL;
 
 	if (md->queue->elevator)
-		return 1;
+		return 0;
 
 	/* Fully initialize the queue */
 	q = blk_init_allocated_queue(md->queue, dm_request_fn, NULL);
 	if (!q)
-		return 0;
+		return -EINVAL;
 
 	/* disable dm_request_fn's merge heuristic by default */
 	md->seq_rq_merge_deadline_usecs = 0;
 
 	md->queue = q;
-	dm_init_md_queue(md);
+	dm_init_old_md_queue(md);
 	blk_queue_softirq_done(md->queue, dm_softirq_done);
 	blk_queue_prep_rq(md->queue, dm_prep_fn);
 
-	/* Also initialize the request-based DM worker thread */
-	init_kthread_worker(&md->kworker);
-	md->kworker_task = kthread_run(kthread_worker_fn, &md->kworker,
-				       "kdmwork-%s", dm_device_name(md));
+	init_rq_based_worker_thread(md);
 
 	elv_register_queue(md->queue);
 
-	return 1;
+	return 0;
+}
+
+static int dm_mq_init_request(void *data, struct request *rq,
+			      unsigned int hctx_idx, unsigned int request_idx,
+			      unsigned int numa_node)
+{
+	struct mapped_device *md = data;
+	struct dm_rq_target_io *tio = blk_mq_rq_to_pdu(rq);
+
+	/*
+	 * Must initialize md member of tio, otherwise it won't
+	 * be available in dm_mq_queue_rq.
+	 */
+	tio->md = md;
+
+	return 0;
+}
+
+static int dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
+			  const struct blk_mq_queue_data *bd)
+{
+	struct request *rq = bd->rq;
+	struct dm_rq_target_io *tio = blk_mq_rq_to_pdu(rq);
+	struct mapped_device *md = tio->md;
+	int srcu_idx;
+	struct dm_table *map = dm_get_live_table(md, &srcu_idx);
+	struct dm_target *ti;
+	sector_t pos;
+
+	/* always use block 0 to find the target for flushes for now */
+	pos = 0;
+	if (!(rq->cmd_flags & REQ_FLUSH))
+		pos = blk_rq_pos(rq);
+
+	ti = dm_table_find_target(map, pos);
+	if (!dm_target_is_valid(ti)) {
+		dm_put_live_table(md, srcu_idx);
+		DMERR_LIMIT("request attempted access beyond the end of device");
+		/*
+		 * Must perform setup, that rq_completed() requires,
+		 * before returning BLK_MQ_RQ_QUEUE_ERROR
+		 */
+		dm_start_request(md, rq);
+		return BLK_MQ_RQ_QUEUE_ERROR;
+	}
+	dm_put_live_table(md, srcu_idx);
+
+	if (ti->type->busy && ti->type->busy(ti))
+		return BLK_MQ_RQ_QUEUE_BUSY;
+
+	dm_start_request(md, rq);
+
+	/* Init tio using md established in .init_request */
+	init_tio(tio, rq, md);
+
+	/* Establish tio->ti before queuing work (map_tio_request) */
+	tio->ti = ti;
+	queue_kthread_work(&md->kworker, &tio->work);
+
+	return BLK_MQ_RQ_QUEUE_OK;
+}
+
+static struct blk_mq_ops dm_mq_ops = {
+	.queue_rq = dm_mq_queue_rq,
+	.map_queue = blk_mq_map_queue,
+	.complete = dm_softirq_done,
+	.init_request = dm_mq_init_request,
+};
+
+static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
+{
+	struct request_queue *q;
+	int err;
+
+	memset(&md->tag_set, 0, sizeof(md->tag_set));
+	md->tag_set.ops = &dm_mq_ops;
+	md->tag_set.queue_depth = BLKDEV_MAX_RQ;
+	md->tag_set.numa_node = NUMA_NO_NODE;
+	md->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
+	md->tag_set.nr_hw_queues = 1;
+	md->tag_set.cmd_size = sizeof(struct dm_rq_target_io);
+	md->tag_set.driver_data = md;
+
+	err = blk_mq_alloc_tag_set(&md->tag_set);
+	if (err)
+		return err;
+
+	q = blk_mq_init_allocated_queue(&md->tag_set, md->queue);
+	if (IS_ERR(q)) {
+		err = PTR_ERR(q);
+		goto out_tag_set;
+	}
+	md->queue = q;
+	dm_init_md_queue(md);
+
+	/* backfill 'mq' sysfs registration normally done in blk_register_queue */
+	blk_mq_register_disk(md->disk);
+
+	init_rq_based_worker_thread(md);
+
+	return 0;
+
+out_tag_set:
+	blk_mq_free_tag_set(&md->tag_set);
+	return err;
 }
 
 /*
···
  */
 int dm_setup_md_queue(struct mapped_device *md)
 {
-	if (dm_md_type_request_based(md)) {
-		if (!dm_init_request_based_queue(md)) {
+	int r;
+	unsigned md_type = dm_get_md_type(md);
+
+	switch (md_type) {
+	case DM_TYPE_REQUEST_BASED:
+		r = dm_init_request_based_queue(md);
+		if (r) {
 			DMWARN("Cannot initialize queue for request-based mapped device");
-			return -EINVAL;
+			return r;
 		}
-	} else {
-		/* bio-based specific initialization */
+		break;
+	case DM_TYPE_MQ_REQUEST_BASED:
+		r = dm_init_request_based_blk_mq_queue(md);
+		if (r) {
+			DMWARN("Cannot initialize queue for request-based blk-mq mapped device");
+			return r;
+		}
+		break;
+	case DM_TYPE_BIO_BASED:
+		dm_init_old_md_queue(md);
 		blk_queue_make_request(md->queue, dm_make_request);
 		blk_queue_merge_bvec(md->queue, dm_merge_bvec);
+		break;
 	}
 
 	return 0;

include/uapi/linux/dm-ioctl.h

···
 #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 */