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

Do not allocate the io_pool mempool for blk-mq request-based DM
(DM_TYPE_MQ_REQUEST_BASED) in dm_alloc_rq_mempools().

Also refine __bind_mempools() to have more precise awareness of which
mempools each type of DM device uses -- avoids mempool churn when
reloading DM tables (particularly for DM_TYPE_REQUEST_BASED).
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 (!t->mempools)
-		return -ENOMEM;
+	if (IS_ERR(t->mempools))
+		return PTR_ERR(t->mempools);
 
 	return 0;
 }

drivers/md/dm.c

@@ -2323,39 +2323,52 @@ 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);
 
-	if (md->bs) {
-		/* The md already has necessary mempools. */
-		if (dm_table_get_type(t) == DM_TYPE_BIO_BASED) {
+	switch (filter_md_type(dm_table_get_type(t), md)) {
+	case DM_TYPE_BIO_BASED:
+		if (md->bs && md->io_pool) {
 			/*
+			 * 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;
 		}
-		/*
-		 * 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;
+		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;
 	}
 
+	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);
@@ -2734,14 +2747,6 @@ 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
  */
@@ -3463,7 +3468,7 @@ struct dm_md_mempools *dm_alloc_bio_mempools(unsigned integrity,
 
 	pools = kzalloc(sizeof(*pools), GFP_KERNEL);
 	if (!pools)
-		return NULL;
+		return ERR_PTR(-ENOMEM);
 
 	front_pad = roundup(per_bio_data_size, __alignof__(struct dm_target_io)) +
 		offsetof(struct dm_target_io, clone);
@@ -3482,24 +3487,26 @@ struct dm_md_mempools *dm_alloc_bio_mempools(unsigned integrity,
 	return pools;
 out:
 	dm_free_md_mempools(pools);
-	return NULL;
+	return ERR_PTR(-ENOMEM);
 }
 
 struct dm_md_mempools *dm_alloc_rq_mempools(struct mapped_device *md,
 					    unsigned type)
 {
-	unsigned int pool_size = dm_get_reserved_rq_based_ios();
+	unsigned int pool_size;
 	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 NULL;
+		return ERR_PTR(-ENOMEM);
 
-	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->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)
@@ -3508,7 +3515,7 @@ struct dm_md_mempools *dm_alloc_rq_mempools(struct mapped_device *md,
 	return pools;
 out:
 	dm_free_md_mempools(pools);
-	return NULL;
+	return ERR_PTR(-ENOMEM);
 }
 
 void dm_free_md_mempools(struct dm_md_mempools *pools)