nvme: add a nvme_ns_head_multipath helper

Move the multipath gendisk out of #ifdef CONFIG_NVME_MULTIPATH and add
a new nvme_ns_head_multipath that uses it to check if a ns_head has
a multipath device associated with it.
Signed-off-by: Minwoo Im <minwoo.im.dev@gmail.com>
[hch: added the IS_ENABLED, converted a few existing users]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Javier González <javier.gonz@samsung.com>
Reviewed-by: Chaitanya Kulkarni <chaitanya.kulkarni@wdc.com>

drivers/nvme/host/core.c

@@ -1873,11 +1873,9 @@ static int nvme_open(struct block_device *bdev, fmode_t mode)
 {
 	struct nvme_ns *ns = bdev->bd_disk->private_data;
 
-#ifdef CONFIG_NVME_MULTIPATH
 	/* should never be called due to GENHD_FL_HIDDEN */
-	if (WARN_ON_ONCE(ns->head->disk))
+	if (WARN_ON_ONCE(nvme_ns_head_multipath(ns->head)))
 		goto fail;
-#endif
 	if (!kref_get_unless_zero(&ns->kref))
 		goto fail;
 	if (!try_module_get(ns->ctrl->ops->module))
@@ -2215,8 +2213,7 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_id_ns *id)
 			return ret;
 	}
 
-#ifdef CONFIG_NVME_MULTIPATH
-	if (ns->head->disk) {
+	if (nvme_ns_head_multipath(ns->head)) {
 		blk_mq_freeze_queue(ns->head->disk->queue);
 		nvme_update_disk_info(ns->head->disk, ns, id);
 		blk_stack_limits(&ns->head->disk->queue->limits,
@@ -2224,7 +2221,6 @@ static int nvme_update_ns_info(struct nvme_ns *ns, struct nvme_id_ns *id)
 		blk_queue_update_readahead(ns->head->disk->queue);
 		blk_mq_unfreeze_queue(ns->head->disk->queue);
 	}
-#endif
 	return 0;
 
 out_unfreeze:

drivers/nvme/host/nvme.h

@@ -413,8 +413,8 @@ struct nvme_ns_head {
 	bool			shared;
 	int			instance;
 	struct nvme_effects_log *effects;
-#ifdef CONFIG_NVME_MULTIPATH
 	struct gendisk		*disk;
+#ifdef CONFIG_NVME_MULTIPATH
 	struct bio_list		requeue_list;
 	spinlock_t		requeue_lock;
 	struct work_struct	requeue_work;
@@ -425,6 +425,11 @@ struct nvme_ns_head {
 #endif
 };
 
+static inline bool nvme_ns_head_multipath(struct nvme_ns_head *head)
+{
+	return IS_ENABLED(CONFIG_NVME_MULTIPATH) && head->disk;
+}
+
 enum nvme_ns_features {
 	NVME_NS_EXT_LBAS = 1 << 0, /* support extended LBA format */
 	NVME_NS_METADATA_SUPPORTED = 1 << 1, /* support getting generated md */