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Date:   Wed, 14 Apr 2021 12:15:40 +0200
From:   Michal Hocko <mhocko@suse.com>
To:     Muchun Song <songmuchun@bytedance.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
Subject: Re: [External] Re: [PATCH 2/7] mm: memcontrol: bail out early when
 !mm in get_mem_cgroup_from_mm
Message-ID: <YHbAzL9ls6EBsB3L@dhcp22.suse.cz>
References: <20210413065153.63431-1-songmuchun@bytedance.com>
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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.

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

Thanks!

-- 
Michal Hocko
SUSE Labs