memcg: kill memcg refcnt

Now memcg has the same life cycle as its corresponding cgroup.  Kill the
useless refcnt.

Signed-off-by: Li Zefan <lizefan@huawei.com>
Acked-by: Michal Hocko <mhocko@suse.cz>
Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Glauber Costa <glommer@openvz.org>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memcontrol.c

@@ -299,8 +299,6 @@ struct mem_cgroup {
 	bool		oom_lock;
 	atomic_t	under_oom;
 
-	atomic_t	refcnt;
-
 	int	swappiness;
 	/* OOM-Killer disable */
 	int		oom_kill_disable;
@@ -503,8 +501,6 @@ enum res_type {
  */
 static DEFINE_MUTEX(memcg_create_mutex);
 
-static void mem_cgroup_put(struct mem_cgroup *memcg);
-
 static inline
 struct mem_cgroup *mem_cgroup_from_css(struct cgroup_subsys_state *s)
 {
@@ -6238,17 +6234,6 @@ static void free_rcu(struct rcu_head *rcu_head)
 	schedule_work(&memcg->work_freeing);
 }
 
-static void __mem_cgroup_put(struct mem_cgroup *memcg, int count)
-{
-	if (atomic_sub_and_test(count, &memcg->refcnt))
-		call_rcu(&memcg->rcu_freeing, free_rcu);
-}
-
-static void mem_cgroup_put(struct mem_cgroup *memcg)
-{
-	__mem_cgroup_put(memcg, 1);
-}
-
 /*
  * Returns the parent mem_cgroup in memcgroup hierarchy with hierarchy enabled.
  */
@@ -6308,7 +6293,6 @@ mem_cgroup_css_alloc(struct cgroup *cont)
 
 	memcg->last_scanned_node = MAX_NUMNODES;
 	INIT_LIST_HEAD(&memcg->oom_notify);
-	atomic_set(&memcg->refcnt, 1);
 	memcg->move_charge_at_immigrate = 0;
 	mutex_init(&memcg->thresholds_lock);
 	spin_lock_init(&memcg->move_lock);
@@ -6399,7 +6383,7 @@ static void mem_cgroup_css_free(struct cgroup *cont)
 	struct mem_cgroup *memcg = mem_cgroup_from_cont(cont);
 
 	memcg_destroy_kmem(memcg);
-	__mem_cgroup_free(memcg);
+	call_rcu(&memcg->rcu_freeing, free_rcu);
 }
 
 #ifdef CONFIG_MMU