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References: <20210319163821.20704-1-songmuchun@bytedance.com>
 <20210319163821.20704-2-songmuchun@bytedance.com> <YFitwmmLFA24cofE@cmpxchg.org>
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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Tue, 23 Mar 2021 17:18:08 +0800
Message-ID: <CAMZfGtVQPonuhQOWq2D_tZyLGuBQ2auY=SF3vPJXN=iTnp8_1Q@mail.gmail.com>
Subject: Re: [External] Re: [PATCH v5 1/7] mm: memcontrol: slab: fix obtain a
 reference to a freeing memcg
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Roman Gushchin <guro@fb.com>, Michal Hocko <mhocko@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Shakeel Butt <shakeelb@google.com>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Xiongchun duan <duanxiongchun@bytedance.com>
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On Mon, Mar 22, 2021 at 10:46 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
>
> On Sat, Mar 20, 2021 at 12:38:14AM +0800, Muchun Song 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>
>
> Acked-by: Johannes Weiner <hannes@cmpxchg.org>
>
> Good catch! Did you trigger the WARN_ON() in
> percpu_ref_kill_and_confirm() during testing?

No. The race window is very small, it should be difficult to trigger.
When I reviewed the code here, I suddenly realized that there
might be a problem here. Very coincidental.

Thanks.