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Tue, 21 Feb 2023 09:01:10 -0800 (PST) MIME-Version: 1.0 References: <20230220230624.lkobqeagycx7bi7p@google.com> <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev> In-Reply-To: From: Martin Zhao Date: Wed, 22 Feb 2023 01:00:00 +0800 Message-ID: Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations To: Shakeel Butt Cc: Roman Gushchin , 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, tangyeechou@gmail.com Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 3757780038 X-Stat-Signature: t4scy5drs9usuxs1zpot3gdnk9qk6e6h X-HE-Tag: 1676998871-620299 X-HE-Meta: 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 syEvl797 JWU3B6i/6fUlxXGVaXl+9YmjucstyguR1g8GOo+JMxv901VvkqPq8lXlOGJTEF/jcz7fQeclN2DLlc9mII+e82ELGF2E6/q4+Lj2F5z1LLPCI4p2uN3KiOwuz6e8ztVYOM3e7nfpBWFeKaBi80sPHcinB5uR8U+vGFV0ySpItbyRjzZEFaAix68cBDWHhVSZraU4VRzVMxxxIgN5LbSnVA9qK+Oj7e4hrRBRiz1+syk/ZhyEvtNJKP4Xw4jY/1q3Cy24F79W4CVe1yix46wd6SAd1ZYbgTXF3sOIJCIrfDuKVhawh/VSatese7o4mLDwDcgQV87O0lqowkAwRMly6zHRbLf2VDkPDI6Ye87BFMoUwRtnCH1zPFfmMVr1hIHz33r6lRBETZa/uAcXvl4KNj+3oZKMAmYRbIya3zVCXunw34T+qsR1TCFzc7e1zruHV2+v5P1JgWWF1l5VlmREKsKkbKQ== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000002, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: On Tue, Feb 21, 2023 at 2:52 PM Shakeel Butt wrote: > > On Mon, Feb 20, 2023 at 9:17 PM Roman Gushchin = wrote: > > > > > On Feb 20, 2023, at 3:06 PM, Shakeel Butt wrote= : > > > > > > =EF=BB=BFOn Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gushchin wro= te: > > >>> 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 > > >> > > >> Hi Yue! > > >> > > >> I'm curious, have any seen any real issues which your patch is solvi= ng? > > >> 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=E2=80=99s obviously not a big deal to put READ_ONCE()/WRITE_O= NCE() here, but I struggle to imagine a scenario when it will make any diff= erence. IMHO it=E2=80=99s easier to justify a proper atomic operation here,= even if it=E2=80=99s most likely an overkill. > > > > 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= . > > > > Also there are other similar cgroup interfaces without READ_ONCE()/WRIT= E_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.