linux-kernel - Re: [PATCH] mm: change memcg->oom_group access with atomic operations

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Date:   Wed, 22 Feb 2023 01:00:00 +0800
From:   Martin Zhao <findns94@...il.com>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>, linux-mm@...ck.org,
        akpm@...ux-foundation.org, hannes@...xchg.org, mhocko@...nel.org,
        muchun.song@...ux.dev, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, tangyeechou@...il.com
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations

On Tue, Feb 21, 2023 at 2:52 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Mon, Feb 20, 2023 at 9:17 PM Roman Gushchin <roman.gushchin@...ux.dev> wrote:
> >
> > > On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@...gle.com> wrote:
> > >
> > > On Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gushchin wrote:
> > >>> On Mon, Feb 20, 2023 at 11:16:38PM +0800, Yue Zhao wrote:
> > >>> The knob for cgroup v2 memory controller: memory.oom.group
> > >>> will be read and written simultaneously by user space
> > >>> programs, thus we'd better change memcg->oom_group access
> > >>> with atomic operations to avoid concurrency problems.
> > >>>
> > >>> Signed-off-by: Yue Zhao <findns94@...il.com>
> > >>
> > >> Hi Yue!
> > >>
> > >> I'm curious, have any seen any real issues which your patch is solving?
> > >> Can you, please, provide a bit more details.
> > >>
> > >
> > > IMHO such details are not needed. oom_group is being accessed
> > > concurrently and one of them can be a write access. At least
> > > READ_ONCE/WRITE_ONCE is needed here.
> >
> > Needed for what?
>
> For this particular case, documenting such an access. Though I don't
> think there are any architectures which may tear a one byte read/write
> and merging/refetching is not an issue for this.
>
> >
> > I mean it’s obviously not a big deal to put READ_ONCE()/WRITE_ONCE() here, but I struggle to imagine a scenario when it will make any difference. IMHO it’s easier to justify a proper atomic operation here, even if it’s most likely an overkill.
> >
> > My question is very simple: the commit log mentions “… to avoid concurrency problems”, so I wonder what problems are these.
> >
> > Also there are other similar cgroup interfaces without READ_ONCE()/WRITE_ONCE()
>
> Yeah and those are v1 interfaces e.g. oom_kill_disable, swappiness,
> soft_limit. These definitely need [READ|WRITE]_ONCE primitive.
>
> Yue, can you update your patch and convert all accesses to these
> fields through [READ|WRITE]_ONCE ?

Sure, it will take some time to update my patch later. I think most of
the accesses use [READ|WRITE]_ONCE already, only few interfaces need to
update.