memcg: don't need to get a reference to the parent

The cgroup core guarantees it's always safe to access the parent.

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>

1 files changed, 3 insertions, 16 deletions

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 76c0c99b002f..c508258d61a1 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -503,7 +503,6 @@ enum res_type {
  */
 static DEFINE_MUTEX(memcg_create_mutex);
 
-static void mem_cgroup_get(struct mem_cgroup *memcg);
 static void mem_cgroup_put(struct mem_cgroup *memcg);
 
 static inline
@@ -6239,19 +6238,10 @@ static void free_rcu(struct rcu_head *rcu_head)
         schedule_work(&memcg->work_freeing);
 }
 
-static void mem_cgroup_get(struct mem_cgroup *memcg)
-{
-        atomic_inc(&memcg->refcnt);
-}
-
 static void __mem_cgroup_put(struct mem_cgroup *memcg, int count)
 {
-        if (atomic_sub_and_test(count, &memcg->refcnt)) {
-                struct mem_cgroup *parent = parent_mem_cgroup(memcg);
+        if (atomic_sub_and_test(count, &memcg->refcnt))
                 call_rcu(&memcg->rcu_freeing, free_rcu);
-                if (parent)
-                        mem_cgroup_put(parent);
-        }
 }
 
 static void mem_cgroup_put(struct mem_cgroup *memcg)
@@ -6354,12 +6344,9 @@ mem_cgroup_css_online(struct cgroup *cont)
                 res_counter_init(&memcg->kmem, &parent->kmem);
 
                 /*
-                 * We increment refcnt of the parent to ensure that we can
-                 * safely access it on res_counter_charge/uncharge.
-                 * This refcnt will be decremented when freeing this
-                 * mem_cgroup(see mem_cgroup_put).
+                 * No need to take a reference to the parent because cgroup
+                 * core guarantees its existence.
                  */
-                mem_cgroup_get(parent);
         } else {
                 res_counter_init(&memcg->res, NULL);
                 res_counter_init(&memcg->memsw, NULL);