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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 5 Feb 2021 07:59:06 -0800
Message-ID: <CALvZod65SY3yVXSwxO02VCZeEg9KsBqq9_Ph3pq2gfQ0eH=kFw@mail.gmail.com>
Subject: Re: [External] Re: [PATCH] mm: memcontrol: remove rcu_read_lock from get_mem_cgroup_from_page
To:     Michal Hocko <mhocko@suse.com>, Roman Gushchin <guro@fb.com>
Cc:     Muchun Song <songmuchun@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Cgroups <cgroups@vger.kernel.org>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
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+Cc Roman

On Fri, Feb 5, 2021 at 2:49 AM Michal Hocko <mhocko@suse.com> wrote:
>
[snip]
> > > > Also, css_get is enough because page
> > > > has a reference to the memcg.
> > >
> > > tryget used to be there to guard against offlined memcg but we have
> > > concluded this is impossible in this path. tryget stayed there to catch
> > > some unexpected cases IIRC.
> >
> > Yeah, it can catch some unexpected cases. But why is this path
> > special so that we need a tryget?
>
> I do not remember details and the changelog of that change is not
> explicit but I suspect it was just because this one could trigger as
> there are external callers to memcg. Is this protection needed? I am not
> sure, this is for you to justify if you want to remove it.
>

It used to be css_tryget_online() which was changed to css_tryget()
and from the discussion at [1], it seemed css_get() would be enough
but we took a safer route.

Anyways, I think we can either take the page_memcg_rcu() route or put
explicit restrictions with page lock or lock_page_memcg() to guarantee
page and memcg binding. I don't have a strong opinion either way but I
think removing restrictions in future for new use-cases will be much
harder, so, page_memcg_rcu() approach seems more appropriate at least
for now.

[1] https://lore.kernel.org/linux-mm/CALvZod5pAv=u8L2Tgk0hDY7XAiiF2dvjC1omQ5BSfzFu_2zSXA@mail.gmail.com/