linux-kernel - [PATCH v3] mm: memcg/slab: Fix return child memcg objcg for root memcg

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Date:   Fri, 30 Oct 2020 00:44:29 +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 v3] mm: memcg/slab: Fix return child memcg objcg for root memcg

Consider the following memcg hierarchy.

                    root
                   /    \
                  A      B

If we failed to get the reference on objcg of memcg A, 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>
Acked-by: Roman Gushchin <guro@...com>
---
 changelog in v3:
 1. Fix commit grammar, thanks to Shakeel.

 changelog in v2:
 1. Do not use a comparison with the root_mem_cgroup

 mm/memcontrol.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 1337775b04f3..8c8b4c3ed5a0 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2961,6 +2961,7 @@ __always_inline struct obj_cgroup *get_obj_cgroup_from_current(void)
 		objcg = rcu_dereference(memcg->objcg);
 		if (objcg && obj_cgroup_tryget(objcg))
 			break;
+		objcg = NULL;
 	}
 	rcu_read_unlock();
 
-- 
2.20.1