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Date:   Fri, 22 Mar 2019 22:49:46 +0000
From:   Chris Down <chris@chrisdown.name>
To:     Roman Gushchin <guro@fb.com>
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>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "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
Message-ID: <20190322224946.GA12527@chrisdown.name>
References: <20190228213050.GA28211@chrisdown.name>
 <20190322160307.GA3316@chrisdown.name>
 <20190322222907.GA17496@tower.DHCP.thefacebook.com>
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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:

low reclaim case:

    In [1]: cgroup_size=50; lruvec_size=10; protection=2000; baseline=0; lruvec_size * cgroup_size // protection - baseline
    Out[1]: 0

normal case:

    In [2]: cgroup_size=3000; lruvec_size=10; protection=2000; baseline=lruvec_size; lruvec_size * cgroup_size // protection - baseline
    Out[2]: 5