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Date: Mon, 1 Jun 2015 15:12:15 +0200
From: Michal Hocko <mhocko@suse.cz>
Subject: Re: [PATCH] mm/oom: Suppress unnecessary "sharing same memory"
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Message-ID: <20150601131215.GI7147@dhcp22.suse.cz>
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To: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Cc: linux-mm@kvack.org

On Mon 01-06-15 22:04:28, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > > Likewise, move do_send_sig_info(SIGKILL, victim) to before
> > > mark_oom_victim(victim) in case for_each_process() took very long time,
> > > for the OOM victim can abuse ALLOC_NO_WATERMARKS by TIF_MEMDIE via e.g.
> > > memset() in user space until SIGKILL is delivered.
> > 
> > This is unrelated and I believe even not necessary.
> 
> Why unnecessary? If serial console is configured and printing a series of
> "Kill process %d (%s) sharing same memory" took a few seconds, the OOM
> victim can consume all memory via malloc() + memset(), can't it?

Can? You are generating one corner case after another. All of them
without actually showing it can happen in the real life. There are
million+1 corner cases possible yet we would prefer to handle those that
have changes to happen in the real life. So let's focus on the real life
scenarios.

> What to do if the OOM victim cannot die immediately after consuming
> all memory? I think that sending SIGKILL before setting TIF_MEMDIE
> helps reducing consumption of memory reserves.

I think that SIGKILL before or after mark_oom_victim has close to zero
effect. If you think that we should send SIGKILL before looking for
tasks sharing mm then why not - BUT AGAIN A SEPARATE PATCH WITH A
JUSTIFICATION please.
-- 
Michal Hocko
SUSE Labs

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