md/raid5: ensure we create a unique name for kmem_cache when mddev has no gendisk

We will shortly allow md devices with no gendisk (they are attached to
a dm-target instead).  That will cause mdname() to return 'mdX'.
There is one place where mdname really needs to be unique: when
creating the name for a slab cache.
So in that case, if there is no gendisk, you the address of the mddev
formatted in HEX to provide a unique name.
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -1337,10 +1337,14 @@ static int grow_stripes(raid5_conf_t *conf, int num)
 	struct kmem_cache *sc;
 	int devs = max(conf->raid_disks, conf->previous_raid_disks);
 
+	if (conf->mddev->gendisk)
 		sprintf(conf->cache_name[0],
 			"raid%d-%s", conf->level, mdname(conf->mddev));
-	sprintf(conf->cache_name[1],
-		"raid%d-%s-alt", conf->level, mdname(conf->mddev));
+	else
+		sprintf(conf->cache_name[0],
+			"raid%d-%p", conf->level, conf->mddev);
+	sprintf(conf->cache_name[1], "%s-alt", conf->cache_name[0]);
+
 	conf->active_name = 0;
 	sc = kmem_cache_create(conf->cache_name[conf->active_name],
 			       sizeof(struct stripe_head)+(devs-1)*sizeof(struct r5dev),

drivers/md/raid5.h

@@ -388,7 +388,7 @@ struct raid5_private_data {
 	 * two caches.
 	 */
 	int			active_name;
-	char			cache_name[2][20];
+	char			cache_name[2][32];
 	struct kmem_cache		*slab_cache; /* for allocating stripes */
 
 	int			seq_flush, seq_write;