dm: remove support for stacking dm-mq on .request_fn device(s)

Remove all fiddley code that propped up this support for a blk-mq
request-queue ontop of all .request_fn devices.

Testing has proven this niche request-based dm-mq mode to be buggy, when
testing fault tolerance with DM multipath, and there is no point trying
to preserve it.

Should help improve efficiency of pure dm-mq code and make code
maintenance less delicate.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-mpath.c

@@ -418,7 +418,10 @@ static int __multipath_map(struct dm_target *ti, struct request *clone,
 	spin_unlock_irq(&m->lock);
 
 	if (clone) {
-		/* Old request-based interface: allocated clone is passed in */
+		/*
+		 * Old request-based interface: allocated clone is passed in.
+		 * Used by: .request_fn stacked on .request_fn path(s).
+		 */
 		clone->q = bdev_get_queue(bdev);
 		clone->rq_disk = bdev->bd_disk;
 		clone->cmd_flags |= REQ_FAILFAST_TRANSPORT;

drivers/md/dm.c

@@ -1141,11 +1141,6 @@ static void free_rq_clone(struct request *clone)
 	else if (!md->queue->mq_ops)
 		/* request_fn queue stacked on request_fn queue(s) */
 		free_clone_request(md, clone);
-	/*
-	 * NOTE: for the blk-mq queue stacked on request_fn queue(s) case:
-	 * no need to call free_clone_request() because we leverage blk-mq by
-	 * allocating the clone at the end of the blk-mq pdu (see: clone_rq)
-	 */
 
 	if (!md->queue->mq_ops)
 		free_rq_tio(tio);
@@ -1866,24 +1861,18 @@ static struct request *clone_rq(struct request *rq, struct mapped_device *md,
 				struct dm_rq_target_io *tio, gfp_t gfp_mask)
 {
 	/*
-	 * Do not allocate a clone if tio->clone was already set
-	 * (see: dm_mq_queue_rq).
+	 * Create clone for use with .request_fn request_queue
 	 */
-	bool alloc_clone = !tio->clone;
 	struct request *clone;
 
-	if (alloc_clone) {
-		clone = alloc_clone_request(md, gfp_mask);
-		if (!clone)
-			return NULL;
-	} else
-		clone = tio->clone;
+	clone = alloc_clone_request(md, gfp_mask);
+	if (!clone)
+		return NULL;
 
 	blk_rq_init(NULL, clone);
 	if (setup_clone(clone, rq, tio, gfp_mask)) {
 		/* -ENOMEM */
-		if (alloc_clone)
-			free_clone_request(md, clone);
+		free_clone_request(md, clone);
 		return NULL;
 	}
 
@@ -2692,22 +2681,12 @@ static int dm_mq_queue_rq(struct blk_mq_hw_ctx *hctx,
 	 */
 	tio->ti = ti;
 
-	/*
-	 * Both the table and md type cannot change after initial table load
-	 */
-	if (dm_get_md_type(md) == 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);
-		(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) {
-			/* Undo dm_start_request() before requeuing */
-			rq_end_stats(md, rq);
-			rq_completed(md, rq_data_dir(rq), false);
-			return BLK_MQ_RQ_QUEUE_BUSY;
-		}
+	/* Direct call is fine since .queue_rq allows allocations */
+	if (map_request(tio, rq, md) == DM_MAPIO_REQUEUE) {
+		/* Undo dm_start_request() before requeuing */
+		rq_end_stats(md, rq);
+		rq_completed(md, rq_data_dir(rq), false);
+		return BLK_MQ_RQ_QUEUE_BUSY;
 	}
 
 	return BLK_MQ_RQ_QUEUE_OK;
@@ -2726,6 +2705,11 @@ static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
 	struct request_queue *q;
 	int err;
 
+	if (dm_get_md_type(md) == DM_TYPE_REQUEST_BASED) {
+		DMERR("request-based dm-mq may only be stacked on blk-mq device(s)");
+		return -EINVAL;
+	}
+
 	md->tag_set = kzalloc(sizeof(struct blk_mq_tag_set), GFP_KERNEL);
 	if (!md->tag_set)
 		return -ENOMEM;
@@ -2738,10 +2722,6 @@ static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
 	md->tag_set->driver_data = md;
 
 	md->tag_set->cmd_size = sizeof(struct dm_rq_target_io);
-	if (md_type == DM_TYPE_REQUEST_BASED) {
-		/* put the memory for non-blk-mq clone at the end of the pdu */
-		md->tag_set->cmd_size += sizeof(struct request);
-	}
 
 	err = blk_mq_alloc_tag_set(md->tag_set);
 	if (err)
@@ -2758,9 +2738,6 @@ static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
 	/* backfill 'mq' sysfs registration normally done in blk_register_queue */
 	blk_mq_register_disk(md->disk);
 
-	if (md_type == DM_TYPE_REQUEST_BASED)
-		init_rq_based_worker_thread(md);
-
 	return 0;
 
 out_tag_set: