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Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations
From:   Muchun Song <muchun.song@linux.dev>
In-Reply-To: <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev>
Date:   Tue, 21 Feb 2023 15:22:32 +0800
Cc:     Shakeel Butt <shakeelb@google.com>, Yue Zhao <findns94@gmail.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, cgroups@vger.kernel.org,
        linux-kernel@vger.kernel.org
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Message-Id: <61CA5FFC-547B-4F56-9238-C9096223BF80@linux.dev>
References: <20230220230624.lkobqeagycx7bi7p@google.com>
 <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev>
To:     Roman Gushchin <roman.gushchin@linux.dev>
Precedence: bulk


> On Feb 21, 2023, at 13:17, Roman Gushchin <roman.gushchin@linux.dev> =
wrote:
>=20
>> On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@google.com> =
wrote:
>>=20
>> =EF=BB=BFOn 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.
>>>>=20
>>>> Signed-off-by: Yue Zhao <findns94@gmail.com>
>>>=20
>>> Hi Yue!
>>>=20
>>> I'm curious, have any seen any real issues which your patch is =
solving?
>>> Can you, please, provide a bit more details.
>>>=20
>>=20
>> 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.
>=20
> Needed for what?
>=20
> I mean it=E2=80=99s 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=E2=80=99s easier to justify a =
proper atomic operation here, even if it=E2=80=99s most likely an =
overkill.
>=20
> My question is very simple: the commit log mentions =E2=80=9C=E2=80=A6 =
to avoid concurrency problems=E2=80=9D, so I wonder what problems are =
these.

I think there is no difference in the assembly code between them in most
cases. The only intention that I can think of is to avoid the potential
complaint (data race) emitted by KCSAN.

>=20
> Also there are other similar cgroup interfaces without =
READ_ONCE()/WRITE_ONCE().

If we decide to fix, then we should fix all.

Thanks.

>=20
> Thanks!