Revert "dm: do not allocate any mempools for blk-mq request-based DM"

This reverts commit cbc4e3c1.
Reported-by: Junichi Nomura <j-nomura@ce.jp.nec.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-table.c

@@ -964,8 +964,8 @@ static int dm_table_alloc_md_mempools(struct dm_table *t, struct mapped_device *
 		return -EINVAL;
 	}
 
-	if (IS_ERR(t->mempools))
-		return PTR_ERR(t->mempools);
+	if (!t->mempools)
+		return -ENOMEM;
 
 	return 0;
 }

drivers/md/dm.c

@@ -2349,52 +2349,39 @@ static void free_dev(struct mapped_device *md)
 	kfree(md);
 }
 
-static unsigned filter_md_type(unsigned type, struct mapped_device *md)
-{
-	if (type == DM_TYPE_BIO_BASED)
-		return type;
-
-	return !md->use_blk_mq ? DM_TYPE_REQUEST_BASED : DM_TYPE_MQ_REQUEST_BASED;
-}
-
 static void __bind_mempools(struct mapped_device *md, struct dm_table *t)
 {
 	struct dm_md_mempools *p = dm_table_get_md_mempools(t);
 
-	switch (filter_md_type(dm_table_get_type(t), md)) {
-	case DM_TYPE_BIO_BASED:
-		if (md->bs && md->io_pool) {
+	if (md->bs) {
+		/* The md already has necessary mempools. */
+		if (dm_table_get_type(t) == DM_TYPE_BIO_BASED) {
 			/*
-			 * This bio-based md already has necessary mempools.
 			 * Reload bioset because front_pad may have changed
 			 * because a different table was loaded.
 			 */
 			bioset_free(md->bs);
 			md->bs = p->bs;
 			p->bs = NULL;
-			goto out;
 		}
-		break;
-	case DM_TYPE_REQUEST_BASED:
-		if (md->rq_pool && md->io_pool)
-			/*
-			 * This request-based md already has necessary mempools.
-			 */
-			goto out;
-		break;
-	case DM_TYPE_MQ_REQUEST_BASED:
-		BUG_ON(p); /* No mempools needed */
-		return;
+		/*
+		 * There's no need to reload with request-based dm
+		 * because the size of front_pad doesn't change.
+		 * Note for future: If you are to reload bioset,
+		 * prep-ed requests in the queue may refer
+		 * to bio from the old bioset, so you must walk
+		 * through the queue to unprep.
+		 */
+		goto out;
 	}
 
-	BUG_ON(!p || md->io_pool || md->rq_pool || md->bs);
-
 	md->io_pool = p->io_pool;
 	p->io_pool = NULL;
 	md->rq_pool = p->rq_pool;
 	p->rq_pool = NULL;
 	md->bs = p->bs;
 	p->bs = NULL;
+
 out:
 	/* mempool bind completed, no longer need any mempools in the table */
 	dm_table_free_md_mempools(t);
@@ -2774,6 +2761,14 @@ static int dm_init_request_based_blk_mq_queue(struct mapped_device *md)
 	return err;
 }
 
+static unsigned filter_md_type(unsigned type, struct mapped_device *md)
+{
+	if (type == DM_TYPE_BIO_BASED)
+		return type;
+
+	return !md->use_blk_mq ? DM_TYPE_REQUEST_BASED : DM_TYPE_MQ_REQUEST_BASED;
+}
+
 /*
  * Setup the DM device's queue based on md's type
  */
@@ -3495,7 +3490,7 @@ struct dm_md_mempools *dm_alloc_bio_mempools(unsigned integrity,
 
 	pools = kzalloc(sizeof(*pools), GFP_KERNEL);
 	if (!pools)
-		return ERR_PTR(-ENOMEM);
+		return NULL;
 
 	front_pad = roundup(per_bio_data_size, __alignof__(struct dm_target_io)) +
 		offsetof(struct dm_target_io, clone);
@@ -3514,26 +3509,24 @@ struct dm_md_mempools *dm_alloc_bio_mempools(unsigned integrity,
 	return pools;
 out:
 	dm_free_md_mempools(pools);
-	return ERR_PTR(-ENOMEM);
+	return NULL;
 }
 
 struct dm_md_mempools *dm_alloc_rq_mempools(struct mapped_device *md,
 					    unsigned type)
 {
-	unsigned int pool_size;
+	unsigned int pool_size = dm_get_reserved_rq_based_ios();
 	struct dm_md_mempools *pools;
 
-	if (filter_md_type(type, md) == DM_TYPE_MQ_REQUEST_BASED)
-		return NULL; /* No mempools needed */
-
-	pool_size = dm_get_reserved_rq_based_ios();
 	pools = kzalloc(sizeof(*pools), GFP_KERNEL);
 	if (!pools)
-		return ERR_PTR(-ENOMEM);
+		return NULL;
 
-	pools->rq_pool = mempool_create_slab_pool(pool_size, _rq_cache);
-	if (!pools->rq_pool)
-		goto out;
+	if (filter_md_type(type, md) == DM_TYPE_REQUEST_BASED) {
+		pools->rq_pool = mempool_create_slab_pool(pool_size, _rq_cache);
+		if (!pools->rq_pool)
+			goto out;
+	}
 
 	pools->io_pool = mempool_create_slab_pool(pool_size, _rq_tio_cache);
 	if (!pools->io_pool)
@@ -3542,7 +3535,7 @@ struct dm_md_mempools *dm_alloc_rq_mempools(struct mapped_device *md,
 	return pools;
 out:
 	dm_free_md_mempools(pools);
-	return ERR_PTR(-ENOMEM);
+	return NULL;
 }
 
 void dm_free_md_mempools(struct dm_md_mempools *pools)