linux-kernel - Re: [PATCH REBASED] mm, memcg: Make scan aggression always exclude protection

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Date:   Fri, 22 Mar 2019 23:01:02 +0000
From:   Roman Gushchin <guro@...com>
To:     Chris Down <chris@...isdown.name>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>, Tejun Heo <tj@...nel.org>,
        Dennis Zhou <dennis@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH REBASED] mm, memcg: Make scan aggression always exclude
 protection

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 list
> > (due to clamp() below).
> 
> Are you certain? If so, I don't see what you mean. This is how the code
> looks in Linus' tree after the fixups:
> 
>    unsigned long cgroup_size = mem_cgroup_size(memcg);
>    unsigned long baseline = 0;
> 
>    if (!sc->memcg_low_reclaim)
>            baseline = lruvec_size;
>    scan = lruvec_size * cgroup_size / protection - baseline;

> 
> 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 = max(cgroup_size, protection);

The old one was racy.

Thanks!