memcg: fix double free and make refcnt sane

1. Fix double-free BUG in error route of mem_cgroup_create().
    mem_cgroup_free() itself frees per-zone-info.
 2. Making refcnt of memcg simple.
    Add 1 refcnt at creation and call free when refcnt goes down to 0.

Reviewed-by: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -2092,14 +2092,10 @@ static struct mem_cgroup *mem_cgroup_alloc(void)
  * Removal of cgroup itself succeeds regardless of refs from swap.
  */
 
-static void mem_cgroup_free(struct mem_cgroup *mem)
+static void __mem_cgroup_free(struct mem_cgroup *mem)
 {
 	int node;
 
-	if (atomic_read(&mem->refcnt) > 0)
-		return;
-
-
 	for_each_node_state(node, N_POSSIBLE)
 		free_mem_cgroup_per_zone_info(mem, node);
 
@@ -2116,11 +2112,8 @@ static void mem_cgroup_get(struct mem_cgroup *mem)
 
 static void mem_cgroup_put(struct mem_cgroup *mem)
 {
-	if (atomic_dec_and_test(&mem->refcnt)) {
-		if (!mem->obsolete)
-			return;
-		mem_cgroup_free(mem);
-	}
+	if (atomic_dec_and_test(&mem->refcnt))
+		__mem_cgroup_free(mem);
 }
 
 
@@ -2170,12 +2163,10 @@ mem_cgroup_create(struct cgroup_subsys *ss, struct cgroup *cont)
 
 	if (parent)
 		mem->swappiness = get_swappiness(parent);
-
+	atomic_set(&mem->refcnt, 1);
 	return &mem->css;
 free_out:
-	for_each_node_state(node, N_POSSIBLE)
-		free_mem_cgroup_per_zone_info(mem, node);
-	mem_cgroup_free(mem);
+	__mem_cgroup_free(mem);
 	return ERR_PTR(-ENOMEM);
 }
 
@@ -2190,7 +2181,7 @@ static void mem_cgroup_pre_destroy(struct cgroup_subsys *ss,
 static void mem_cgroup_destroy(struct cgroup_subsys *ss,
 				struct cgroup *cont)
 {
-	mem_cgroup_free(mem_cgroup_from_cont(cont));
+	mem_cgroup_put(mem_cgroup_from_cont(cont));
 }
 
 static int mem_cgroup_populate(struct cgroup_subsys *ss,