linux-kernel - [PATCH 1/5] mm: memcg/slab: Fix return child memcg objcg for root memcg

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Date:   Tue, 27 Oct 2020 16:02:52 +0800
From:   Muchun Song <songmuchun@...edance.com>
To:     hannes@...xchg.org, mhocko@...nel.org, vdavydov.dev@...il.com,
        akpm@...ux-foundation.org, shakeelb@...gle.com, guro@...com,
        iamjoonsoo.kim@....com, laoar.shao@...il.com, chris@...isdown.name,
        christian.brauner@...ntu.com, peterz@...radead.org,
        mingo@...nel.org, keescook@...omium.org, tglx@...utronix.de,
        esyr@...hat.com, surenb@...gle.com, areber@...hat.com,
        elver@...gle.com
Cc:     linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
        linux-mm@...ck.org, Muchun Song <songmuchun@...edance.com>
Subject: [PATCH 1/5] mm: memcg/slab: Fix return child memcg objcg for root memcg

Consider the following memcg hierarchy.

                    root
                   /    \
                  A      B

If we get the objcg of memcg A failed, the get_obj_cgroup_from_current
can return the wrong objcg for the root memcg.

Fixes: bf4f059954dc ("mm: memcg/slab: obj_cgroup API")
Signed-off-by: Muchun Song <songmuchun@...edance.com>
---
 mm/memcontrol.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1337775b04f3..fcbd79c5023e 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2945,7 +2945,7 @@ struct mem_cgroup *mem_cgroup_from_obj(void *p)
 
 __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
 {
-	struct obj_cgroup *objcg = NULL;
+	struct obj_cgroup *objcg;
 	struct mem_cgroup *memcg;
 
 	if (memcg_kmem_bypass())
@@ -2964,6 +2964,9 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
 	}
 	rcu_read_unlock();
 
+	if (memcg == root_mem_cgroup)
+		return NULL;
+
 	return objcg;
 }
 
-- 
2.20.1