scsi: add support for a blk-mq based I/O path.

This patch adds support for an alternate I/O path in the scsi midlayer
which uses the blk-mq infrastructure instead of the legacy request code.

Use of blk-mq is fully transparent to drivers, although for now a host
template field is provided to opt out of blk-mq usage in case any unforseen
incompatibilities arise.

In general replacing the legacy request code with blk-mq is a simple and
mostly mechanical transformation.  The biggest exception is the new code
that deals with the fact the I/O submissions in blk-mq must happen from
process context, which slightly complicates the I/O completion handler.
The second biggest differences is that blk-mq is build around the concept
of preallocated requests that also include driver specific data, which
in SCSI context means the scsi_cmnd structure.  This completely avoids
dynamic memory allocations for the fast path through I/O submission.

Due the preallocated requests the MQ code path exclusively uses the
host-wide shared tag allocator instead of a per-LUN one.  This only
affects drivers actually using the block layer provided tag allocator
instead of their own.  Unlike the old path blk-mq always provides a tag,
although drivers don't have to use it.

For now the blk-mq path is disable by defauly and must be enabled using
the "use_blk_mq" module parameter.  Once the remaining work in the block
layer to make blk-mq more suitable for slow devices is complete I hope
to make it the default and eventually even remove the old code path.

Based on the earlier scsi-mq prototype by Nicholas Bellinger.

Thanks to Bart Van Assche and Robert Elliot for testing, benchmarking and
various sugestions and code contributions.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Webb Scales <webbnh@hp.com>
Acked-by: Jens Axboe <axboe@kernel.dk>
Tested-by: Bart Van Assche <bvanassche@acm.org>
Tested-by: Robert Elliott <elliott@hp.com>

drivers/scsi/hosts.c

@@ -213,9 +213,24 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 		goto fail;
 	}
 
-	error = scsi_setup_command_freelist(shost);
+	if (shost_use_blk_mq(shost)) {
+		error = scsi_mq_setup_tags(shost);
 		if (error)
 			goto fail;
+	}
+
+	/*
+	 * Note that we allocate the freelist even for the MQ case for now,
+	 * as we need a command set aside for scsi_reset_provider.  Having
+	 * the full host freelist and one command available for that is a
+	 * little heavy-handed, but avoids introducing a special allocator
+	 * just for this.  Eventually the structure of scsi_reset_provider
+	 * will need a major overhaul.
+	 */
+	error = scsi_setup_command_freelist(shost);
+	if (error)
+		goto out_destroy_tags;
+
 
 	if (!shost->shost_gendev.parent)
 		shost->shost_gendev.parent = dev ? dev : &platform_bus;
@@ -226,7 +241,7 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 
 	error = device_add(&shost->shost_gendev);
 	if (error)
-		goto out;
+		goto out_destroy_freelist;
 
 	pm_runtime_set_active(&shost->shost_gendev);
 	pm_runtime_enable(&shost->shost_gendev);
@@ -279,8 +294,11 @@ int scsi_add_host_with_dma(struct Scsi_Host *shost, struct device *dev,
 	device_del(&shost->shost_dev);
  out_del_gendev:
 	device_del(&shost->shost_gendev);
- out:
+ out_destroy_freelist:
 	scsi_destroy_command_freelist(shost);
+ out_destroy_tags:
+	if (shost_use_blk_mq(shost))
+		scsi_mq_destroy_tags(shost);
  fail:
 	return error;
 }
@@ -309,8 +327,13 @@ static void scsi_host_dev_release(struct device *dev)
 	}
 
 	scsi_destroy_command_freelist(shost);
+	if (shost_use_blk_mq(shost)) {
+		if (shost->tag_set.tags)
+			scsi_mq_destroy_tags(shost);
+	} else {
 		if (shost->bqt)
 			blk_free_tags(shost->bqt);
+	}
 
 	kfree(shost->shost_data);
 
@@ -436,6 +459,8 @@ struct Scsi_Host *scsi_host_alloc(struct scsi_host_template *sht, int privsize)
 	else
 		shost->dma_boundary = 0xffffffff;
 
+	shost->use_blk_mq = scsi_use_blk_mq && !shost->hostt->disable_blk_mq;
+
 	device_initialize(&shost->shost_gendev);
 	dev_set_name(&shost->shost_gendev, "host%d", shost->host_no);
 	shost->shost_gendev.bus = &scsi_bus_type;

drivers/scsi/scsi.c

@@ -805,7 +805,7 @@ void scsi_adjust_queue_depth(struct scsi_device *sdev, int tagged, int tags)
 	 * is more IO than the LLD's can_queue (so there are not enuogh
 	 * tags) request_fn's host queue ready check will handle it.
 	 */
-	if (!sdev->host->bqt) {
+	if (!shost_use_blk_mq(sdev->host) && !sdev->host->bqt) {
 		if (blk_queue_tagged(sdev->request_queue) &&
 		    blk_queue_resize_tags(sdev->request_queue, tags) != 0)
 			goto out;
@@ -1361,6 +1361,9 @@ MODULE_LICENSE("GPL");
 module_param(scsi_logging_level, int, S_IRUGO|S_IWUSR);
 MODULE_PARM_DESC(scsi_logging_level, "a bit mask of logging levels");
 
+bool scsi_use_blk_mq = false;
+module_param_named(use_blk_mq, scsi_use_blk_mq, bool, S_IWUSR | S_IRUGO);
+
 static int __init init_scsi(void)
 {
 	int error;

drivers/scsi/scsi_lib.c

 /*
- *  scsi_lib.c Copyright (C) 1999 Eric Youngdale
+ * Copyright (C) 1999 Eric Youngdale
+ * Copyright (C) 2014 Christoph Hellwig
  *
  *  SCSI queueing library.
  *      Initial versions: Eric Youngdale (eric@andante.org).
@@ -20,6 +21,7 @@
 #include <linux/delay.h>
 #include <linux/hardirq.h>
 #include <linux/scatterlist.h>
+#include <linux/blk-mq.h>
 
 #include <scsi/scsi.h>
 #include <scsi/scsi_cmnd.h>
@@ -113,6 +115,16 @@ scsi_set_blocked(struct scsi_cmnd *cmd, int reason)
 	}
 }
 
+static void scsi_mq_requeue_cmd(struct scsi_cmnd *cmd)
+{
+	struct scsi_device *sdev = cmd->device;
+	struct request_queue *q = cmd->request->q;
+
+	blk_mq_requeue_request(cmd->request);
+	blk_mq_kick_requeue_list(q);
+	put_device(&sdev->sdev_gendev);
+}
+
 /**
  * __scsi_queue_insert - private queue insertion
  * @cmd: The SCSI command being requeued
@@ -150,6 +162,10 @@ static void __scsi_queue_insert(struct scsi_cmnd *cmd, int reason, int unbusy)
 	 * before blk_cleanup_queue() finishes.
 	 */
 	cmd->result = 0;
+	if (q->mq_ops) {
+		scsi_mq_requeue_cmd(cmd);
+		return;
+	}
 	spin_lock_irqsave(q->queue_lock, flags);
 	blk_requeue_request(q, cmd->request);
 	kblockd_schedule_work(&device->requeue_work);
@@ -308,6 +324,14 @@ void scsi_device_unbusy(struct scsi_device *sdev)
 	atomic_dec(&sdev->device_busy);
 }
 
+static void scsi_kick_queue(struct request_queue *q)
+{
+	if (q->mq_ops)
+		blk_mq_start_hw_queues(q);
+	else
+		blk_run_queue(q);
+}
+
 /*
  * Called for single_lun devices on IO completion. Clear starget_sdev_user,
  * and call blk_run_queue for all the scsi_devices on the target -
@@ -332,7 +356,7 @@ static void scsi_single_lun_run(struct scsi_device *current_sdev)
 	 * but in most cases, we will be first. Ideally, each LU on the
 	 * target would get some limited time or requests on the target.
 	 */
-	blk_run_queue(current_sdev->request_queue);
+	scsi_kick_queue(current_sdev->request_queue);
 
 	spin_lock_irqsave(shost->host_lock, flags);
 	if (starget->starget_sdev_user)
@@ -345,7 +369,7 @@ static void scsi_single_lun_run(struct scsi_device *current_sdev)
 			continue;
 
 		spin_unlock_irqrestore(shost->host_lock, flags);
-		blk_run_queue(sdev->request_queue);
+		scsi_kick_queue(sdev->request_queue);
 		spin_lock_irqsave(shost->host_lock, flags);
 	
 		scsi_device_put(sdev);
@@ -435,7 +459,7 @@ static void scsi_starved_list_run(struct Scsi_Host *shost)
 			continue;
 		spin_unlock_irqrestore(shost->host_lock, flags);
 
-		blk_run_queue(slq);
+		scsi_kick_queue(slq);
 		blk_put_queue(slq);
 
 		spin_lock_irqsave(shost->host_lock, flags);
@@ -466,6 +490,9 @@ static void scsi_run_queue(struct request_queue *q)
 	if (!list_empty(&sdev->host->starved_list))
 		scsi_starved_list_run(sdev->host);
 
+	if (q->mq_ops)
+		blk_mq_start_stopped_hw_queues(q, false);
+	else
 		blk_run_queue(q);
 }
 
@@ -564,25 +591,72 @@ static struct scatterlist *scsi_sg_alloc(unsigned int nents, gfp_t gfp_mask)
 	return mempool_alloc(sgp->pool, gfp_mask);
 }
 
-static void scsi_free_sgtable(struct scsi_data_buffer *sdb)
+static void scsi_free_sgtable(struct scsi_data_buffer *sdb, bool mq)
 {
-	__sg_free_table(&sdb->table, SCSI_MAX_SG_SEGMENTS, false, scsi_sg_free);
+	if (mq && sdb->table.nents <= SCSI_MAX_SG_SEGMENTS)
+		return;
+	__sg_free_table(&sdb->table, SCSI_MAX_SG_SEGMENTS, mq, scsi_sg_free);
 }
 
 static int scsi_alloc_sgtable(struct scsi_data_buffer *sdb, int nents,
-			      gfp_t gfp_mask)
+			      gfp_t gfp_mask, bool mq)
 {
+	struct scatterlist *first_chunk = NULL;
 	int ret;
 
 	BUG_ON(!nents);
 
+	if (mq) {
+		if (nents <= SCSI_MAX_SG_SEGMENTS) {
+			sdb->table.nents = nents;
+			sg_init_table(sdb->table.sgl, sdb->table.nents);
+			return 0;
+		}
+		first_chunk = sdb->table.sgl;
+	}
+
 	ret = __sg_alloc_table(&sdb->table, nents, SCSI_MAX_SG_SEGMENTS,
-			       NULL, gfp_mask, scsi_sg_alloc);
+			       first_chunk, gfp_mask, scsi_sg_alloc);
 	if (unlikely(ret))
-		scsi_free_sgtable(sdb);
+		scsi_free_sgtable(sdb, mq);
 	return ret;
 }
 
+static void scsi_uninit_cmd(struct scsi_cmnd *cmd)
+{
+	if (cmd->request->cmd_type == REQ_TYPE_FS) {
+		struct scsi_driver *drv = scsi_cmd_to_driver(cmd);
+
+		if (drv->uninit_command)
+			drv->uninit_command(cmd);
+	}
+}
+
+static void scsi_mq_free_sgtables(struct scsi_cmnd *cmd)
+{
+	if (cmd->sdb.table.nents)
+		scsi_free_sgtable(&cmd->sdb, true);
+	if (cmd->request->next_rq && cmd->request->next_rq->special)
+		scsi_free_sgtable(cmd->request->next_rq->special, true);
+	if (scsi_prot_sg_count(cmd))
+		scsi_free_sgtable(cmd->prot_sdb, true);
+}
+
+static void scsi_mq_uninit_cmd(struct scsi_cmnd *cmd)
+{
+	struct scsi_device *sdev = cmd->device;
+	unsigned long flags;
+
+	BUG_ON(list_empty(&cmd->list));
+
+	scsi_mq_free_sgtables(cmd);
+	scsi_uninit_cmd(cmd);
+
+	spin_lock_irqsave(&sdev->list_lock, flags);
+	list_del_init(&cmd->list);
+	spin_unlock_irqrestore(&sdev->list_lock, flags);
+}
+
 /*
  * Function:    scsi_release_buffers()
  *
@@ -602,19 +676,19 @@ static int scsi_alloc_sgtable(struct scsi_data_buffer *sdb, int nents,
 static void scsi_release_buffers(struct scsi_cmnd *cmd)
 {
 	if (cmd->sdb.table.nents)
-		scsi_free_sgtable(&cmd->sdb);
+		scsi_free_sgtable(&cmd->sdb, false);
 
 	memset(&cmd->sdb, 0, sizeof(cmd->sdb));
 
 	if (scsi_prot_sg_count(cmd))
-		scsi_free_sgtable(cmd->prot_sdb);
+		scsi_free_sgtable(cmd->prot_sdb, false);
 }
 
 static void scsi_release_bidi_buffers(struct scsi_cmnd *cmd)
 {
 	struct scsi_data_buffer *bidi_sdb = cmd->request->next_rq->special;
 
-	scsi_free_sgtable(bidi_sdb);
+	scsi_free_sgtable(bidi_sdb, false);
 	kmem_cache_free(scsi_sdb_cache, bidi_sdb);
 	cmd->request->next_rq->special = NULL;
 }
@@ -625,8 +699,6 @@ static bool scsi_end_request(struct request *req, int error,
 	struct scsi_cmnd *cmd = req->special;
 	struct scsi_device *sdev = cmd->device;
 	struct request_queue *q = sdev->request_queue;
-	unsigned long flags;
-
 
 	if (blk_update_request(req, error, bytes))
 		return true;
@@ -639,6 +711,28 @@ static bool scsi_end_request(struct request *req, int error,
 	if (blk_queue_add_random(q))
 		add_disk_randomness(req->rq_disk);
 
+	if (req->mq_ctx) {
+		/*
+		 * In the MQ case the command gets freed by __blk_mq_end_io,
+		 * so we have to do all cleanup that depends on it earlier.
+		 *
+		 * We also can't kick the queues from irq context, so we
+		 * will have to defer it to a workqueue.
+		 */
+		scsi_mq_uninit_cmd(cmd);
+
+		__blk_mq_end_io(req, error);
+
+		if (scsi_target(sdev)->single_lun ||
+		    !list_empty(&sdev->host->starved_list))
+			kblockd_schedule_work(&sdev->requeue_work);
+		else
+			blk_mq_start_stopped_hw_queues(q, true);
+
+		put_device(&sdev->sdev_gendev);
+	} else {
+		unsigned long flags;
+
 		spin_lock_irqsave(q->queue_lock, flags);
 		blk_finish_request(req, error);
 		spin_unlock_irqrestore(q->queue_lock, flags);
@@ -647,6 +741,8 @@ static bool scsi_end_request(struct request *req, int error,
 			scsi_release_bidi_buffers(cmd);
 		scsi_release_buffers(cmd);
 		scsi_next_command(cmd);
+	}
+
 	return false;
 }
 
@@ -953,8 +1049,14 @@ void scsi_io_completion(struct scsi_cmnd *cmd, unsigned int good_bytes)
 		/* Unprep the request and put it back at the head of the queue.
 		 * A new command will be prepared and issued.
 		 */
+		if (q->mq_ops) {
+			cmd->request->cmd_flags &= ~REQ_DONTPREP;
+			scsi_mq_uninit_cmd(cmd);
+			scsi_mq_requeue_cmd(cmd);
+		} else {
 			scsi_release_buffers(cmd);
 			scsi_requeue_command(q, cmd);
+		}
 		break;
 	case ACTION_RETRY:
 		/* Retry the same command immediately */
@@ -976,9 +1078,8 @@ static int scsi_init_sgtable(struct request *req, struct scsi_data_buffer *sdb,
 	 * If sg table allocation fails, requeue request later.
 	 */
 	if (unlikely(scsi_alloc_sgtable(sdb, req->nr_phys_segments,
-					gfp_mask))) {
+					gfp_mask, req->mq_ctx != NULL)))
 		return BLKPREP_DEFER;
-	}
 
 	/* 
 	 * Next, walk the list, and fill in the addresses and sizes of
@@ -1006,6 +1107,7 @@ int scsi_init_io(struct scsi_cmnd *cmd, gfp_t gfp_mask)
 {
 	struct scsi_device *sdev = cmd->device;
 	struct request *rq = cmd->request;
+	bool is_mq = (rq->mq_ctx != NULL);
 	int error;
 
 	BUG_ON(!rq->nr_phys_segments);
@@ -1015,15 +1117,19 @@ int scsi_init_io(struct scsi_cmnd *cmd, gfp_t gfp_mask)
 		goto err_exit;
 
 	if (blk_bidi_rq(rq)) {
-		struct scsi_data_buffer *bidi_sdb = kmem_cache_zalloc(
-			scsi_sdb_cache, GFP_ATOMIC);
+		if (!rq->q->mq_ops) {
+			struct scsi_data_buffer *bidi_sdb =
+				kmem_cache_zalloc(scsi_sdb_cache, GFP_ATOMIC);
 			if (!bidi_sdb) {
 				error = BLKPREP_DEFER;
 				goto err_exit;
 			}
 
 			rq->next_rq->special = bidi_sdb;
-		error = scsi_init_sgtable(rq->next_rq, bidi_sdb, GFP_ATOMIC);
+		}
+
+		error = scsi_init_sgtable(rq->next_rq, rq->next_rq->special,
+					  GFP_ATOMIC);
 		if (error)
 			goto err_exit;
 	}
@@ -1035,7 +1141,7 @@ int scsi_init_io(struct scsi_cmnd *cmd, gfp_t gfp_mask)
 		BUG_ON(prot_sdb == NULL);
 		ivecs = blk_rq_count_integrity_sg(rq->q, rq->bio);
 
-		if (scsi_alloc_sgtable(prot_sdb, ivecs, gfp_mask)) {
+		if (scsi_alloc_sgtable(prot_sdb, ivecs, gfp_mask, is_mq)) {
 			error = BLKPREP_DEFER;
 			goto err_exit;
 		}
@@ -1049,13 +1155,16 @@ int scsi_init_io(struct scsi_cmnd *cmd, gfp_t gfp_mask)
 		cmd->prot_sdb->table.nents = count;
 	}
 
-	return BLKPREP_OK ;
-
+	return BLKPREP_OK;
 err_exit:
+	if (is_mq) {
+		scsi_mq_free_sgtables(cmd);
+	} else {
 		scsi_release_buffers(cmd);
 		cmd->request->special = NULL;
 		scsi_put_command(cmd);
 		put_device(&sdev->sdev_gendev);
+	}
 	return error;
 }
 EXPORT_SYMBOL(scsi_init_io);
@@ -1266,13 +1375,7 @@ static int scsi_prep_fn(struct request_queue *q, struct request *req)
 
 static void scsi_unprep_fn(struct request_queue *q, struct request *req)
 {
-	if (req->cmd_type == REQ_TYPE_FS) {
-		struct scsi_cmnd *cmd = req->special;
-		struct scsi_driver *drv = scsi_cmd_to_driver(cmd);
-
-		if (drv->uninit_command)
-			drv->uninit_command(cmd);
-	}
+	scsi_uninit_cmd(req->special);
 }
 
 /*
@@ -1295,6 +1398,10 @@ static inline int scsi_dev_queue_ready(struct request_queue *q,
 		 * unblock after device_blocked iterates to zero
 		 */
 		if (atomic_dec_return(&sdev->device_blocked) > 0) {
+			/*
+			 * For the MQ case we take care of this in the caller.
+			 */
+			if (!q->mq_ops)
 				blk_delay_queue(q, SCSI_QUEUE_DELAY);
 			goto out_dec;
 		}
@@ -1671,6 +1778,180 @@ static void scsi_request_fn(struct request_queue *q)
 		blk_delay_queue(q, SCSI_QUEUE_DELAY);
 }
 
+static inline int prep_to_mq(int ret)
+{
+	switch (ret) {
+	case BLKPREP_OK:
+		return 0;
+	case BLKPREP_DEFER:
+		return BLK_MQ_RQ_QUEUE_BUSY;
+	default:
+		return BLK_MQ_RQ_QUEUE_ERROR;
+	}
+}
+
+static int scsi_mq_prep_fn(struct request *req)
+{
+	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(req);
+	struct scsi_device *sdev = req->q->queuedata;
+	struct Scsi_Host *shost = sdev->host;
+	unsigned char *sense_buf = cmd->sense_buffer;
+	struct scatterlist *sg;
+
+	memset(cmd, 0, sizeof(struct scsi_cmnd));
+
+	req->special = cmd;
+
+	cmd->request = req;
+	cmd->device = sdev;
+	cmd->sense_buffer = sense_buf;
+
+	cmd->tag = req->tag;
+
+	req->cmd = req->__cmd;
+	cmd->cmnd = req->cmd;
+	cmd->prot_op = SCSI_PROT_NORMAL;
+
+	INIT_LIST_HEAD(&cmd->list);
+	INIT_DELAYED_WORK(&cmd->abort_work, scmd_eh_abort_handler);
+	cmd->jiffies_at_alloc = jiffies;
+
+	/*
+	 * XXX: cmd_list lookups are only used by two drivers, try to get
+	 * rid of this list in common code.
+	 */
+	spin_lock_irq(&sdev->list_lock);
+	list_add_tail(&cmd->list, &sdev->cmd_list);
+	spin_unlock_irq(&sdev->list_lock);
+
+	sg = (void *)cmd + sizeof(struct scsi_cmnd) + shost->hostt->cmd_size;
+	cmd->sdb.table.sgl = sg;
+
+	if (scsi_host_get_prot(shost)) {
+		cmd->prot_sdb = (void *)sg +
+			shost->sg_tablesize * sizeof(struct scatterlist);
+		memset(cmd->prot_sdb, 0, sizeof(struct scsi_data_buffer));
+
+		cmd->prot_sdb->table.sgl =
+			(struct scatterlist *)(cmd->prot_sdb + 1);
+	}
+
+	if (blk_bidi_rq(req)) {
+		struct request *next_rq = req->next_rq;
+		struct scsi_data_buffer *bidi_sdb = blk_mq_rq_to_pdu(next_rq);
+
+		memset(bidi_sdb, 0, sizeof(struct scsi_data_buffer));
+		bidi_sdb->table.sgl =
+			(struct scatterlist *)(bidi_sdb + 1);
+
+		next_rq->special = bidi_sdb;
+	}
+
+	return scsi_setup_cmnd(sdev, req);
+}
+
+static void scsi_mq_done(struct scsi_cmnd *cmd)
+{
+	trace_scsi_dispatch_cmd_done(cmd);
+	blk_mq_complete_request(cmd->request);
+}
+
+static int scsi_queue_rq(struct blk_mq_hw_ctx *hctx, struct request *req)
+{
+	struct request_queue *q = req->q;
+	struct scsi_device *sdev = q->queuedata;
+	struct Scsi_Host *shost = sdev->host;
+	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(req);
+	int ret;
+	int reason;
+
+	ret = prep_to_mq(scsi_prep_state_check(sdev, req));
+	if (ret)
+		goto out;
+
+	ret = BLK_MQ_RQ_QUEUE_BUSY;
+	if (!get_device(&sdev->sdev_gendev))
+		goto out;
+
+	if (!scsi_dev_queue_ready(q, sdev))
+		goto out_put_device;
+	if (!scsi_target_queue_ready(shost, sdev))
+		goto out_dec_device_busy;
+	if (!scsi_host_queue_ready(q, shost, sdev))
+		goto out_dec_target_busy;
+
+	if (!(req->cmd_flags & REQ_DONTPREP)) {
+		ret = prep_to_mq(scsi_mq_prep_fn(req));
+		if (ret)
+			goto out_dec_host_busy;
+		req->cmd_flags |= REQ_DONTPREP;
+	}
+
+	scsi_init_cmd_errh(cmd);
+	cmd->scsi_done = scsi_mq_done;
+
+	reason = scsi_dispatch_cmd(cmd);
+	if (reason) {
+		scsi_set_blocked(cmd, reason);
+		ret = BLK_MQ_RQ_QUEUE_BUSY;
+		goto out_dec_host_busy;
+	}
+
+	return BLK_MQ_RQ_QUEUE_OK;
+
+out_dec_host_busy:
+	atomic_dec(&shost->host_busy);
+out_dec_target_busy:
+	if (scsi_target(sdev)->can_queue > 0)
+		atomic_dec(&scsi_target(sdev)->target_busy);
+out_dec_device_busy:
+	atomic_dec(&sdev->device_busy);
+out_put_device:
+	put_device(&sdev->sdev_gendev);
+out:
+	switch (ret) {
+	case BLK_MQ_RQ_QUEUE_BUSY:
+		blk_mq_stop_hw_queue(hctx);
+		if (atomic_read(&sdev->device_busy) == 0 &&
+		    !scsi_device_blocked(sdev))
+			blk_mq_delay_queue(hctx, SCSI_QUEUE_DELAY);
+		break;
+	case BLK_MQ_RQ_QUEUE_ERROR:
+		/*
+		 * Make sure to release all allocated ressources when
+		 * we hit an error, as we will never see this command
+		 * again.
+		 */
+		if (req->cmd_flags & REQ_DONTPREP)
+			scsi_mq_uninit_cmd(cmd);
+		break;
+	default:
+		break;
+	}
+	return ret;
+}
+
+static int scsi_init_request(void *data, struct request *rq,
+		unsigned int hctx_idx, unsigned int request_idx,
+		unsigned int numa_node)
+{
+	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(rq);
+
+	cmd->sense_buffer = kzalloc_node(SCSI_SENSE_BUFFERSIZE, GFP_KERNEL,
+			numa_node);
+	if (!cmd->sense_buffer)
+		return -ENOMEM;
+	return 0;
+}
+
+static void scsi_exit_request(void *data, struct request *rq,
+		unsigned int hctx_idx, unsigned int request_idx)
+{
+	struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(rq);
+
+	kfree(cmd->sense_buffer);
+}
+
 static u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
 {
 	struct device *host_dev;
@@ -1692,16 +1973,10 @@ static u64 scsi_calculate_bounce_limit(struct Scsi_Host *shost)
 	return bounce_limit;
 }
 
-struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
-					 request_fn_proc *request_fn)
+static void __scsi_init_queue(struct Scsi_Host *shost, struct request_queue *q)
 {
-	struct request_queue *q;
 	struct device *dev = shost->dma_dev;
 
-	q = blk_init_queue(request_fn, NULL);
-	if (!q)
-		return NULL;
-
 	/*
 	 * this limit is imposed by hardware restrictions
 	 */
@@ -1732,7 +2007,17 @@ struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
 	 * blk_queue_update_dma_alignment() later.
 	 */
 	blk_queue_dma_alignment(q, 0x03);
+}
+
+struct request_queue *__scsi_alloc_queue(struct Scsi_Host *shost,
+					 request_fn_proc *request_fn)
+{
+	struct request_queue *q;
 
+	q = blk_init_queue(request_fn, NULL);
+	if (!q)
+		return NULL;
+	__scsi_init_queue(shost, q);
 	return q;
 }
 EXPORT_SYMBOL(__scsi_alloc_queue);
@@ -1753,6 +2038,55 @@ struct request_queue *scsi_alloc_queue(struct scsi_device *sdev)
 	return q;
 }
 
+static struct blk_mq_ops scsi_mq_ops = {
+	.map_queue	= blk_mq_map_queue,
+	.queue_rq	= scsi_queue_rq,
+	.complete	= scsi_softirq_done,
+	.timeout	= scsi_times_out,
+	.init_request	= scsi_init_request,
+	.exit_request	= scsi_exit_request,
+};
+
+struct request_queue *scsi_mq_alloc_queue(struct scsi_device *sdev)
+{
+	sdev->request_queue = blk_mq_init_queue(&sdev->host->tag_set);
+	if (IS_ERR(sdev->request_queue))
+		return NULL;
+
+	sdev->request_queue->queuedata = sdev;
+	__scsi_init_queue(sdev->host, sdev->request_queue);
+	return sdev->request_queue;
+}
+
+int scsi_mq_setup_tags(struct Scsi_Host *shost)
+{
+	unsigned int cmd_size, sgl_size, tbl_size;
+
+	tbl_size = shost->sg_tablesize;
+	if (tbl_size > SCSI_MAX_SG_SEGMENTS)
+		tbl_size = SCSI_MAX_SG_SEGMENTS;
+	sgl_size = tbl_size * sizeof(struct scatterlist);
+	cmd_size = sizeof(struct scsi_cmnd) + shost->hostt->cmd_size + sgl_size;
+	if (scsi_host_get_prot(shost))
+		cmd_size += sizeof(struct scsi_data_buffer) + sgl_size;
+
+	memset(&shost->tag_set, 0, sizeof(shost->tag_set));
+	shost->tag_set.ops = &scsi_mq_ops;
+	shost->tag_set.nr_hw_queues = 1;
+	shost->tag_set.queue_depth = shost->can_queue;
+	shost->tag_set.cmd_size = cmd_size;
+	shost->tag_set.numa_node = NUMA_NO_NODE;
+	shost->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_SG_MERGE;
+	shost->tag_set.driver_data = shost;
+
+	return blk_mq_alloc_tag_set(&shost->tag_set);
+}
+
+void scsi_mq_destroy_tags(struct Scsi_Host *shost)
+{
+	blk_mq_free_tag_set(&shost->tag_set);
+}
+
 /*
  * Function:    scsi_block_requests()
  *
@@ -2498,9 +2832,13 @@ scsi_internal_device_block(struct scsi_device *sdev)
 	 * block layer from calling the midlayer with this device's
 	 * request queue. 
 	 */
+	if (q->mq_ops) {
+		blk_mq_stop_hw_queues(q);
+	} else {
 		spin_lock_irqsave(q->queue_lock, flags);
 		blk_stop_queue(q);
 		spin_unlock_irqrestore(q->queue_lock, flags);
+	}
 
 	return 0;
 }
@@ -2546,9 +2884,13 @@ scsi_internal_device_unblock(struct scsi_device *sdev,
 		 sdev->sdev_state != SDEV_OFFLINE)
 		return -EINVAL;
 
+	if (q->mq_ops) {
+		blk_mq_start_stopped_hw_queues(q, false);
+	} else {
 		spin_lock_irqsave(q->queue_lock, flags);
 		blk_start_queue(q);
 		spin_unlock_irqrestore(q->queue_lock, flags);
+	}
 
 	return 0;
 }

drivers/scsi/scsi_priv.h

@@ -88,6 +88,9 @@ extern void scsi_next_command(struct scsi_cmnd *cmd);
 extern void scsi_io_completion(struct scsi_cmnd *, unsigned int);
 extern void scsi_run_host_queues(struct Scsi_Host *shost);
 extern struct request_queue *scsi_alloc_queue(struct scsi_device *sdev);
+extern struct request_queue *scsi_mq_alloc_queue(struct scsi_device *sdev);
+extern int scsi_mq_setup_tags(struct Scsi_Host *shost);
+extern void scsi_mq_destroy_tags(struct Scsi_Host *shost);
 extern int scsi_init_queue(void);
 extern void scsi_exit_queue(void);
 struct request_queue;

drivers/scsi/scsi_scan.c

@@ -273,6 +273,9 @@ static struct scsi_device *scsi_alloc_sdev(struct scsi_target *starget,
 	 */
 	sdev->borken = 1;
 
+	if (shost_use_blk_mq(shost))
+		sdev->request_queue = scsi_mq_alloc_queue(sdev);
+	else
 		sdev->request_queue = scsi_alloc_queue(sdev);
 	if (!sdev->request_queue) {
 		/* release fn is set up in scsi_sysfs_device_initialise, so

drivers/scsi/scsi_sysfs.c

@@ -333,6 +333,7 @@ store_shost_eh_deadline(struct device *dev, struct device_attribute *attr,
 
 static DEVICE_ATTR(eh_deadline, S_IRUGO | S_IWUSR, show_shost_eh_deadline, store_shost_eh_deadline);
 
+shost_rd_attr(use_blk_mq, "%d\n");
 shost_rd_attr(unique_id, "%u\n");
 shost_rd_attr(cmd_per_lun, "%hd\n");
 shost_rd_attr(can_queue, "%hd\n");
@@ -352,6 +353,7 @@ show_host_busy(struct device *dev, struct device_attribute *attr, char *buf)
 static DEVICE_ATTR(host_busy, S_IRUGO, show_host_busy, NULL);
 
 static struct attribute *scsi_sysfs_shost_attrs[] = {
+	&dev_attr_use_blk_mq.attr,
 	&dev_attr_unique_id.attr,
 	&dev_attr_host_busy.attr,
 	&dev_attr_cmd_per_lun.attr,

include/scsi/scsi_host.h

@@ -7,6 +7,7 @@
 #include <linux/workqueue.h>
 #include <linux/mutex.h>
 #include <linux/seq_file.h>
+#include <linux/blk-mq.h>
 #include <scsi/scsi.h>
 
 struct request_queue;
@@ -510,6 +511,9 @@ struct scsi_host_template {
 	 */
 	unsigned int cmd_size;
 	struct scsi_host_cmd_pool *cmd_pool;
+
+	/* temporary flag to disable blk-mq I/O path */
+	bool disable_blk_mq;
 };
 
 /*
@@ -580,7 +584,10 @@ struct Scsi_Host {
 	 * Area to keep a shared tag map (if needed, will be
 	 * NULL if not).
 	 */
+	union {
 		struct blk_queue_tag	*bqt;
+		struct blk_mq_tag_set	tag_set;
+	};
 
 	atomic_t host_busy;		   /* commands actually active on low-level */
 	atomic_t host_blocked;
@@ -672,6 +679,8 @@ struct Scsi_Host {
 	/* The controller does not support WRITE SAME */
 	unsigned no_write_same:1;
 
+	unsigned use_blk_mq:1;
+
 	/*
 	 * Optional work queue to be utilized by the transport
 	 */
@@ -772,6 +781,13 @@ static inline int scsi_host_in_recovery(struct Scsi_Host *shost)
 		shost->tmf_in_progress;
 }
 
+extern bool scsi_use_blk_mq;
+
+static inline bool shost_use_blk_mq(struct Scsi_Host *shost)
+{
+	return shost->use_blk_mq;
+}
+
 extern int scsi_queue_work(struct Scsi_Host *, struct work_struct *);
 extern void scsi_flush_work(struct Scsi_Host *);
 

include/scsi/scsi_tcq.h

@@ -67,7 +67,8 @@ static inline void scsi_activate_tcq(struct scsi_device *sdev, int depth)
 	if (!sdev->tagged_supported)
 		return;
 
-	if (!blk_queue_tagged(sdev->request_queue))
+	if (!shost_use_blk_mq(sdev->host) &&
+	    blk_queue_tagged(sdev->request_queue))
 		blk_queue_init_tags(sdev->request_queue, depth,
 				    sdev->host->bqt);
 
@@ -80,7 +81,8 @@ static inline void scsi_activate_tcq(struct scsi_device *sdev, int depth)
  **/
 static inline void scsi_deactivate_tcq(struct scsi_device *sdev, int depth)
 {
-	if (blk_queue_tagged(sdev->request_queue))
+	if (!shost_use_blk_mq(sdev->host) &&
+	    blk_queue_tagged(sdev->request_queue))
 		blk_queue_free_tags(sdev->request_queue);
 	scsi_adjust_queue_depth(sdev, 0, depth);
 }
@@ -108,6 +110,15 @@ static inline int scsi_populate_tag_msg(struct scsi_cmnd *cmd, char *msg)
 	return 0;
 }
 
+static inline struct scsi_cmnd *scsi_mq_find_tag(struct Scsi_Host *shost,
+		unsigned int hw_ctx, int tag)
+{
+	struct request *req;
+
+	req = blk_mq_tag_to_rq(shost->tag_set.tags[hw_ctx], tag);
+	return req ? (struct scsi_cmnd *)req->special : NULL;
+}
+
 /**
  * scsi_find_tag - find a tagged command by device
  * @SDpnt:	pointer to the ScSI device
@@ -118,10 +129,12 @@ static inline int scsi_populate_tag_msg(struct scsi_cmnd *cmd, char *msg)
  **/
 static inline struct scsi_cmnd *scsi_find_tag(struct scsi_device *sdev, int tag)
 {
-
         struct request *req;
 
         if (tag != SCSI_NO_TAG) {
+		if (shost_use_blk_mq(sdev->host))
+			return scsi_mq_find_tag(sdev->host, 0, tag);
+
         	req = blk_queue_find_tag(sdev->request_queue, tag);
 	        return req ? (struct scsi_cmnd *)req->special : NULL;
 	}
@@ -130,6 +143,7 @@ static inline struct scsi_cmnd *scsi_find_tag(struct scsi_device *sdev, int tag)
 	return sdev->current_cmnd;
 }
 
+
 /**
  * scsi_init_shared_tag_map - create a shared tag map
  * @shost:	the host to share the tag map among all devices
@@ -137,6 +151,12 @@ static inline struct scsi_cmnd *scsi_find_tag(struct scsi_device *sdev, int tag)
  */
 static inline int scsi_init_shared_tag_map(struct Scsi_Host *shost, int depth)
 {
+	/*
+	 * We always have a shared tag map around when using blk-mq.
+	 */
+	if (shost_use_blk_mq(shost))
+		return 0;
+
 	/*
 	 * If the shared tag map isn't already initialized, do it now.
 	 * This saves callers from having to check ->bqt when setting up
@@ -165,6 +185,8 @@ static inline struct scsi_cmnd *scsi_host_find_tag(struct Scsi_Host *shost,
 	struct request *req;
 
 	if (tag != SCSI_NO_TAG) {
+		if (shost_use_blk_mq(shost))
+			return scsi_mq_find_tag(shost, 0, tag);
 		req = blk_map_queue_find_tag(shost->bqt, tag);
 		return req ? (struct scsi_cmnd *)req->special : NULL;
 	}