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From:   Shakeel Butt <shakeelb@google.com>
Date:   Mon, 4 May 2020 06:54:47 -0700
Message-ID: <CALvZod7qEE-k0uoSbUR-+pD2kWshz2SYQNXaVCLPiTTG8sqAmQ@mail.gmail.com>
Subject: Re: [PATCH] memcg: oom: ignore oom warnings from memory.max
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Johannes Weiner <hannes@cmpxchg.org>, Roman Gushchin <guro@fb.com>,
        Greg Thelen <gthelen@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linux MM <linux-mm@kvack.org>,
        Cgroups <cgroups@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Sun, May 3, 2020 at 11:57 PM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Thu 30-04-20 13:20:10, Shakeel Butt wrote:
> > On Thu, Apr 30, 2020 at 12:29 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > On Thu, Apr 30, 2020 at 11:27:12AM -0700, Shakeel Butt wrote:
> > > > Lowering memory.max can trigger an oom-kill if the reclaim does not
> > > > succeed. However if oom-killer does not find a process for killing, it
> > > > dumps a lot of warnings.
> > > >
> > > > Deleting a memcg does not reclaim memory from it and the memory can
> > > > linger till there is a memory pressure. One normal way to proactively
> > > > reclaim such memory is to set memory.max to 0 just before deleting the
> > > > memcg. However if some of the memcg's memory is pinned by others, this
> > > > operation can trigger an oom-kill without any process and thus can log a
> > > > lot un-needed warnings. So, ignore all such warnings from memory.max.
> > >
> > > Can't you set memory.high=0 instead? It does the reclaim portion of
> > > memory.max, without the actual OOM killing that causes you problems.
> >
> > Yes that would work but remote charging concerns me. Remote charging
> > can still happen after the memcg is offlined and at the moment, high
> > reclaim does not work for remote memcg and the usage can go till max
> > or global pressure. This is most probably a misconfiguration and we
> > might not receive the warnings in the log ever. Setting memory.max to
> > 0 will definitely give such warnings.
>
> Can we add a warning for the remote charging on dead memcgs?
>

I don't think we should warn for all remote charging on dead memcgs.
One particular example is the buffer_head which can be allocated
within reclaim context and most probably pages which they are attached
to will be freed soon.

Shakeel