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From:   Muchun Song <songmuchun@bytedance.com>
Date:   Thu, 15 Apr 2021 11:13:16 +0800
Message-ID: <CAMZfGtVGBW-j8_2+dmrD=Bbc2mOXUj4Hg_pjOKi0YrY_OVj1nA@mail.gmail.com>
Subject: Re: [External] Re: [PATCH 2/7] mm: memcontrol: bail out early when
 !mm in get_mem_cgroup_from_mm
To:     Michal Hocko <mhocko@suse.com>
Cc:     guro@fb.com, hannes@cmpxchg.org, akpm@linux-foundation.org,
        shakeelb@google.com, vdavydov.dev@gmail.com,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        duanxiongchun@bytedance.com, fam.zheng@bytedance.com
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On Wed, Apr 14, 2021 at 6:15 PM Michal Hocko <mhocko@suse.com> wrote:
>
> On Wed 14-04-21 18:04:35, Muchun Song wrote:
> > On Wed, Apr 14, 2021 at 5:24 PM Michal Hocko <mhocko@suse.com> wrote:
> > >
> > > On Tue 13-04-21 14:51:48, Muchun Song wrote:
> > > > When mm is NULL, we do not need to hold rcu lock and call css_tryget for
> > > > the root memcg. And we also do not need to check !mm in every loop of
> > > > while. So bail out early when !mm.
> > >
> > > mem_cgroup_charge and other callers unconditionally drop the reference
> > > so how come this does not underflow reference count?
> >
> > For the root memcg, the CSS_NO_REF flag is set, so css_get
> > and css_put do not get or put reference.
>
> Ohh, right you are. I must have forgot about that special case. I am
> pretty sure I (and likely few more) will stumble over that in the future
> again. A small comment explaining that the reference can be safely
> ignore would be helpful.

OK. Will do.

>
> Anyway
> Acked-by: Michal Hocko <mhocko@suse.com>

Thanks.

>
> Thanks!
>
> --
> Michal Hocko
> SUSE Labs