nvme: warn about shared namespaces without CONFIG_NVME_MULTIPATH

Start warning about exposing a namespace as multiple block devices,
and set a fixed deprecation release.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>

drivers/block/loop.c

@@ -2092,6 +2092,7 @@ static void loop_remove(struct loop_device *lo)
 	del_gendisk(lo->lo_disk);
 	blk_cleanup_disk(lo->lo_disk);
 	blk_mq_free_tag_set(&lo->tag_set);
+
 	mutex_lock(&loop_ctl_mutex);
 	idr_remove(&loop_index_idr, lo->lo_number);
 	mutex_unlock(&loop_ctl_mutex);

drivers/nvme/host/core.c

@@ -3855,6 +3855,14 @@ static int nvme_init_ns_head(struct nvme_ns *ns, unsigned nsid,
 					nsid);
 			goto out_put_ns_head;
 		}
+
+		if (!multipath && !list_empty(&head->list)) {
+			dev_warn(ctrl->device,
+				"Found shared namespace %d, but multipathing not supported.\n",
+				nsid);
+			dev_warn_once(ctrl->device,
+				"Support for shared namespaces without CONFIG_NVME_MULTIPATH is deprecated and will be removed in Linux 6.0\n.");
+		}
 	}
 
 	list_add_tail_rcu(&ns->siblings, &head->list);