linux-kernel - Re: [PATCH] mm/oom_kill: wake futex waiters before annihilating victim shared mutex

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Message-ID: <YeVUDVV4Z5ur9Flh@dhcp22.suse.cz>
Date:   Mon, 17 Jan 2022 12:33:33 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     Joel Savitz <jsavitz@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Waiman Long <longman@...hat.com>, linux-mm@...ck.org,
        Nico Pache <npache@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Darren Hart <dvhart@...radead.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        André Almeida <andrealmeid@...labora.com>
Subject: Re: [PATCH] mm/oom_kill: wake futex waiters before annihilating
 victim shared mutex

I have only noticed your email now after replying to v3 so our emails
have crossed.

On Fri 14-01-22 09:39:55, Joel Savitz wrote:
> > What has happened to the oom victim and why it has never exited?
> 
> What appears to happen is that the oom victim is sent SIGKILL by the
> process that triggers the oom while also being marked as an oom
> victim.
> 
> As you mention in your patchset introducing the oom reaper in commit
> aac4536355496 ("mm, oom: introduce oom reaper"), the purpose the the
> oom reaper is to try and free more memory more quickly than it
> otherwise would have been by assuming anonymous or swapped out pages
> won't be needed in the exit path as the owner is already dying.
> However, this assumption is violated by the futex_cleanup() path,
> which needs access to userspace in fetch_robust_entry() when it is
> called in exit_robust_list(). Trace_printk()s in this failure path
> reveal an apparent race between the oom reaper thread reaping the
> victim's mm and the futex_cleanup() path. There may be other ways that
> this race manifests but we have been most consistently able to trace
> that one.

Please let's continue the discussion in the v3 email thread:
http://lkml.kernel.org/r/20220114180135.83308-1-npache@redhat.com
-- 
Michal Hocko
SUSE Labs