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From:   Roman Gushchin <guro@fb.com>
To:     Chris Down <chris@chrisdown.name>
CC:     Andrew Morton <akpm@linux-foundation.org>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>, Tejun Heo <tj@kernel.org>,
        Dennis Zhou <dennis@kernel.org>,
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        "cgroups@vger.kernel.org" <cgroups@vger.kernel.org>,
        "linux-mm@kvack.org" <linux-mm@kvack.org>,
        Kernel Team <Kernel-team@fb.com>
Subject: Re: [PATCH REBASED] mm, memcg: Make scan aggression always exclude
 protection
Thread-Topic: [PATCH REBASED] mm, memcg: Make scan aggression always exclude
 protection
Thread-Index: AQHU4MjBAXhBvPMCI0qZ3+WtTr5x4qYXxmuAgAB7GwCAAAMggA==
Date:   Fri, 22 Mar 2019 23:01:02 +0000
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References: <20190228213050.GA28211@chrisdown.name>
 <20190322160307.GA3316@chrisdown.name>
 <20190322222907.GA17496@tower.DHCP.thefacebook.com>
 <20190322224946.GA12527@chrisdown.name>
In-Reply-To: <20190322224946.GA12527@chrisdown.name>
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On Fri, Mar 22, 2019 at 10:49:46PM +0000, Chris Down wrote:
> Roman Gushchin writes:
> > I've noticed that the old version is just wrong: if cgroup_size is way =
smaller
> > than max(min, low), scan will be set to -lruvec_size.
> > Given that it's unsigned long, we'll end up with scanning the whole lis=
t
> > (due to clamp() below).
>=20
> Are you certain? If so, I don't see what you mean. This is how the code
> looks in Linus' tree after the fixups:
>=20
>    unsigned long cgroup_size =3D mem_cgroup_size(memcg);
>    unsigned long baseline =3D 0;
>=20
>    if (!sc->memcg_low_reclaim)
>            baseline =3D lruvec_size;
>    scan =3D lruvec_size * cgroup_size / protection - baseline;

>=20
> This works correctly as far as I can tell:

I'm blaming the old version, not the new one.

New one is perfectly fine, thanks to these lines:
+                       /* Avoid TOCTOU with earlier protection check */
+                       cgroup_size =3D max(cgroup_size, protection);

The old one was racy.

Thanks!