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References: <20210319163821.20704-1-songmuchun@bytedance.com> <20210319163821.20704-2-songmuchun@bytedance.com>
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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 19 Mar 2021 11:26:54 -0700
Message-ID: <CALvZod73P5QORH58wRi4a2wNnhxCKsGWkFB8Ko79G8SNWr9fhg@mail.gmail.com>
Subject: Re: [PATCH v5 1/7] mm: memcontrol: slab: fix obtain a reference to a
 freeing memcg
To:     Muchun Song <songmuchun@bytedance.com>
Cc:     Roman Gushchin <guro@fb.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Xiongchun duan <duanxiongchun@bytedance.com>
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On Fri, Mar 19, 2021 at 9:38 AM Muchun Song <songmuchun@bytedance.com> wrote:
>
> The rcu_read_lock/unlock only can guarantee that the memcg will not be
> freed, but it cannot guarantee the success of css_get (which is in the
> refill_stock when cached memcg changed) to memcg.
>
>   rcu_read_lock()
>   memcg = obj_cgroup_memcg(old)
>   __memcg_kmem_uncharge(memcg)
>       refill_stock(memcg)
>           if (stock->cached != memcg)
>               // css_get can change the ref counter from 0 back to 1.
>               css_get(&memcg->css)
>   rcu_read_unlock()
>
> This fix is very like the commit:
>
>   eefbfa7fd678 ("mm: memcg/slab: fix use after free in obj_cgroup_charge")
>
> Fix this by holding a reference to the memcg which is passed to the
> __memcg_kmem_uncharge() before calling __memcg_kmem_uncharge().
>
> Fixes: 3de7d4f25a74 ("mm: memcg/slab: optimize objcg stock draining")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Good catch.

Reviewed-by: Shakeel Butt <shakeelb@google.com>