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From: Roman Gushchin MIME-Version: 1.0 Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations Date: Mon, 20 Feb 2023 21:17:25 -0800 Message-Id: <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev> References: <20230220230624.lkobqeagycx7bi7p@google.com> Cc: Yue Zhao , linux-mm@kvack.org, akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, muchun.song@linux.dev, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org In-Reply-To: <20230220230624.lkobqeagycx7bi7p@google.com> To: Shakeel Butt X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Queue-Id: 8FC2A180003 X-Rspamd-Server: rspam01 X-Stat-Signature: wa696s79pam9sy4p7hnhj95pmscsxf68 X-HE-Tag: 1676956660-715485 X-HE-Meta: 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 kD0OLwD3 0rm89S1T7UQlol8+I8vqPBlbn7bsIr3/XmivmoFK2A7wzNPTq8skhygiuXnwEG8UZ6TyJrDHoHXP7gUHxgiM8eJgEWiqmzcReoyXBsU3za7bnAjlN8xDs5a25AMPWRYD/xC0e/XFMEF4z+wtVwWlFHM87hSSw7PBoYCtbfhEfvvSEjeTDnoF2FjGySgitU/hI0RKVHl3LtAJzGLc= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: > On Feb 20, 2023, at 3:06 PM, Shakeel Butt 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 >>=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. Needed for what? 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 i= f it=E2=80=99s most likely an overkill. My question is very simple: the commit log mentions =E2=80=9C=E2=80=A6 to av= oid concurrency problems=E2=80=9D, so I wonder what problems are these. Also there are other similar cgroup interfaces without READ_ONCE()/WRITE_ONC= E(). Thanks!=