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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:cc:to:subject:message-id:date:from :in-reply-to:references:mime-version:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=3usF6y0jP8WdRmsHw7idRFJkuoGf3L15ygmJMO39Keg=; b=l4XYoNjqHwu6y7uIFFalUsiGbxb6Y/8x//xZe+A6gDilpW6xim/AY1/YavMqEM7LoD mHn9u1kEgQzBYUOZKNRz0Yn+4dMp/FBWBfWHlNIRTLXwUIHiIIEzGwKvG3+b03HjbYcm j/uEXFq5vBmZ1cXvCExSDOBuaCfWuoRSp4uvc+Gc1gwYEVhGMWn4ktJWzO010oBKO6on XrJGYDnxnP6YynkTm7mApUk2aqUN9XKl5SgEBjEx+1WK23ZuMwkCTvRhJPsYgLNqVtot c+1wWkE8Pw1FrcK2AoGWilZzBluwOkWUc3l4XHWsy4BSVvqeNv1RHhNgYMQrE4xrzTL3 F/EA== X-Gm-Message-State: AO0yUKWUTwZrEyWsKH+wrz/QZF1vz0xyjAQaT9VG37UdGG98ixQOnND3 3UK5GJyEyzLbNHsRc36EcX4Ed5Hxf5nNnUIBFIPYWw== X-Google-Smtp-Source: AK7set/KssXsBw+UV6ZW5jaXWXgEk7m9tk+TPyEqax5PBIK816L+TDG+tB7u5x98cSyQtCNn8gMxbS2O6rpsFDVFbTk= X-Received: by 2002:a5b:691:0:b0:902:535c:8399 with SMTP id j17-20020a5b0691000000b00902535c8399mr34958ybq.461.1676962341842; Mon, 20 Feb 2023 22:52:21 -0800 (PST) MIME-Version: 1.0 References: <20230220230624.lkobqeagycx7bi7p@google.com> <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev> In-Reply-To: <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev> From: Shakeel Butt Date: Mon, 20 Feb 2023 22:52:10 -0800 Message-ID: Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations To: Roman Gushchin 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 Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam03 X-Stat-Signature: i3x149mr754mpnc888fycqk1ombhsz54 X-Rspamd-Queue-Id: E28311C0006 X-HE-Tag: 1676962342-623609 X-HE-Meta: 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 dmiOJZgf eIM8rON1bmceh2jpugj7Ya1FNDXMZW2jaqjMgoQPhfru/WLkdpmBobdNkl3GVbbX7ekTGGRbVjy1i4QVfkKVa/qhGuJQWuuIGk1W8PDrmA5UHZyQeOtnE/b3I5QgQKwMuS6kDN/eivjoTz80rMPrF1hF4Sb9Qdd5TASLMVnTgEtgfL6VFSLiFkY2yO1YCrvGlOCAbPVUbCmh91CH1c3NKi+dDaR3TmUSuLr4SAp6FCNFgbRfTJXhcFrVuCfKa0XlCf6ZknJ6pdLXtMtH+0dsST1A5jAJJj5Xg1s60HmyensloB5ZlinBVkk5OPeMUmZI1RzxTp/SUdrnRahfDFq1NU/U9xw== 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 Mon, Feb 20, 2023 at 9:17 PM Roman Gushchin w= rote: > > > 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 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 > >> > >> 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=E2=80=99s obviously not a big deal to put READ_ONCE()/WRITE_ONC= E() here, but I struggle to imagine a scenario when it will make any differ= ence. IMHO it=E2=80=99s easier to justify a proper atomic operation here, e= ven 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()/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 ?