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

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Message-ID: <CALvZod55K5zbbVYptq8ud=nKVyU1xceGVf6UcambBZ3BA2TZqA@mail.gmail.com>
Date:   Mon, 20 Feb 2023 22:52:10 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <roman.gushchin@...ux.dev>
Cc:     Yue Zhao <findns94@...il.com>, 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
Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations

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 ?