memcg, kmem: use is_root_cache instead of hard code

Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
Reviewed-by: Pekka Enberg <penberg@kernel.org>
Acked-by: David Rientjes <rientjes@google.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Glauber Costa <glommer@parallels.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -59,6 +59,7 @@
 #include <net/sock.h>
 #include <net/ip.h>
 #include <net/tcp_memcontrol.h>
+#include "slab.h"
 
 #include <asm/uaccess.h>
 
@@ -3131,7 +3132,7 @@ int memcg_update_cache_size(struct kmem_cache *s, int num_groups)
 {
 	struct memcg_cache_params *cur_params = s->memcg_params;
 
-	VM_BUG_ON(s->memcg_params && !s->memcg_params->is_root_cache);
+	VM_BUG_ON(!is_root_cache(s));
 
 	if (num_groups > memcg_limited_groups_array_size) {
 		int i;