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References: <20210223091101.42150-1-songmuchun@bytedance.com>
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From:   Shakeel Butt <shakeelb@google.com>
Date:   Tue, 23 Feb 2021 07:18:46 -0800
Message-ID: <CALvZod6=TYHPC3g3RzX84KeR8p74RH5PdbT+aNy+m5n2Yboskw@mail.gmail.com>
Subject: Re: [PATCH] mm: memcontrol: fix get_active_memcg return value
To:     Muchun Song <songmuchun@bytedance.com>, stable@vger.kernel.org
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Roman Gushchin <guro@fb.com>,
        Cgroups <cgroups@vger.kernel.org>, Linux MM <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Feb 23, 2021 at 1:14 AM Muchun Song <songmuchun@bytedance.com> wrote:
>
> We use a global percpu int_active_memcg variable to store the remote
> memcg when we are in the interrupt context. But get_active_memcg always
> return the current->active_memcg or root_mem_cgroup. The remote memcg
> (set in the interrupt context) is ignored. This is not what we want.
> So fix it.
>
> Fixes: 37d5985c003d ("mm: kmem: prepare remote memcg charging infra for interrupt contexts")
> Signed-off-by: Muchun Song <songmuchun@bytedance.com>

Good catch.
Cc: stable@vger.kernel.org

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

> ---
>  mm/memcontrol.c | 10 +++-------
>  1 file changed, 3 insertions(+), 7 deletions(-)
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index be6bc5044150..bbe25655f7eb 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -1061,13 +1061,9 @@ static __always_inline struct mem_cgroup *get_active_memcg(void)
>
>         rcu_read_lock();
>         memcg = active_memcg();
> -       if (memcg) {
> -               /* current->active_memcg must hold a ref. */
> -               if (WARN_ON_ONCE(!css_tryget(&memcg->css)))
> -                       memcg = root_mem_cgroup;
> -               else
> -                       memcg = current->active_memcg;
> -       }
> +       /* remote memcg must hold a ref. */
> +       if (memcg && WARN_ON_ONCE(!css_tryget(&memcg->css)))
> +               memcg = root_mem_cgroup;
>         rcu_read_unlock();
>
>         return memcg;
> --
> 2.11.0
>