dm: fix request-based DM to not bounce through indirect dm_submit_bio

It is unnecessary to force request-based DM to call into bio-based
dm_submit_bio (via indirect disk->fops->submit_bio) only to have it then
call blk_mq_submit_bio().

Fix this by establishing a request-based DM block_device_operations
(dm_rq_blk_dops, which doesn't have .submit_bio) and update
dm_setup_md_queue() to set md->disk->fops to it for
DM_TYPE_REQUEST_BASED.

Remove DM_TYPE_REQUEST_BASED conditional in dm_submit_bio and unexport
blk_mq_submit_bio.

Fixes: c62b37d9 ("block: move ->make_request_fn to struct block_device_operations")
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

block/blk-mq.c

@@ -2265,7 +2265,6 @@ blk_qc_t blk_mq_submit_bio(struct bio *bio)
 	blk_queue_exit(q);
 	return BLK_QC_T_NONE;
 }
-EXPORT_SYMBOL_GPL(blk_mq_submit_bio); /* only for request based dm */
 
 void blk_mq_free_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 		     unsigned int hctx_idx)

drivers/md/dm.c

@@ -1633,18 +1633,6 @@ static blk_qc_t dm_submit_bio(struct bio *bio)
 	int srcu_idx;
 	struct dm_table *map;
 
-	if (dm_get_md_type(md) == DM_TYPE_REQUEST_BASED) {
-		/*
-		 * We are called with a live reference on q_usage_counter, but
-		 * that one will be released as soon as we return.  Grab an
-		 * extra one as blk_mq_submit_bio expects to be able to consume
-		 * a reference (which lives until the request is freed in case a
-		 * request is allocated).
-		 */
-		percpu_ref_get(&bio->bi_disk->queue->q_usage_counter);
-		return blk_mq_submit_bio(bio);
-	}
-
 	map = dm_get_live_table(md, &srcu_idx);
 	if (unlikely(!map)) {
 		DMERR_LIMIT("%s: mapping table unavailable, erroring io",
@@ -1727,6 +1715,7 @@ static int next_free_minor(int *minor)
 }
 
 static const struct block_device_operations dm_blk_dops;
+static const struct block_device_operations dm_rq_blk_dops;
 static const struct dax_operations dm_dax_ops;
 
 static void dm_wq_work(struct work_struct *work);
@@ -2113,9 +2102,10 @@ int dm_setup_md_queue(struct mapped_device *md, struct dm_table *t)
 
 	switch (type) {
 	case DM_TYPE_REQUEST_BASED:
+		md->disk->fops = &dm_rq_blk_dops;
 		r = dm_mq_init_request_queue(md, t);
 		if (r) {
-			DMERR("Cannot initialize queue for request-based dm-mq mapped device");
+			DMERR("Cannot initialize queue for request-based dm mapped device");
 			return r;
 		}
 		break;
@@ -3095,6 +3085,15 @@ static const struct block_device_operations dm_blk_dops = {
 	.owner = THIS_MODULE
 };
 
+static const struct block_device_operations dm_rq_blk_dops = {
+	.open = dm_blk_open,
+	.release = dm_blk_close,
+	.ioctl = dm_blk_ioctl,
+	.getgeo = dm_blk_getgeo,
+	.pr_ops = &dm_pr_ops,
+	.owner = THIS_MODULE
+};
+
 static const struct dax_operations dm_dax_ops = {
 	.direct_access = dm_dax_direct_access,
 	.dax_supported = dm_dax_supported,