linux-kernel - Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing the robust_list

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Message-ID: <YlAg+Pu7hIw6W+cc@dhcp22.suse.cz>
Date:   Fri, 8 Apr 2022 13:48:08 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Nico Pache <npache@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Rafael Aquini <aquini@...hat.com>,
        Waiman Long <longman@...hat.com>, Baoquan He <bhe@...hat.com>,
        Christoph von Recklinghausen <crecklin@...hat.com>,
        Don Dutile <ddutile@...hat.com>,
        "Herton R . Krzesinski" <herton@...hat.com>,
        David Rientjes <rientjes@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Davidlohr Bueso <dave@...olabs.net>,
        Ingo Molnar <mingo@...hat.com>,
        Joel Savitz <jsavitz@...hat.com>,
        Darren Hart <dvhart@...radead.org>, stable@...nel.org
Subject: Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing
 the robust_list_head

On Fri 08-04-22 07:26:07, Nico Pache wrote:
[...]
> Ok so if i understand that correctly, delaying can have some ugly effects and
> kinda breaks the initial purpose of the OOM reaper?

No, not really. The primary objective of the oom_reaper is to _guaratee_
a forward progress. It is not really meant to be an optimization to
respond to the oom killer faster. The reason the oom_reaper is kicked
off right away is because that was the simplest implementation.

> I personally don't like the delay approach. Especially if we have a better one
> we know is working, and that doesnt add regressions.

Well, I would say that handling futex case more gracefully would be
preferable but my understanding is that this is not all that easy. I am
far from being a futex expert so I will leave that up to Thomas and Peter.

On the other hand delaying oom_reaper is rather straightforward and I do
not think there is a big risk of regressions. Any QoS during OOM is
simply out of the window and the main purpose of the reaper will be
preserved with a timeout as well. I also do agree with Thomas that this
would cover 99% of cases.

> If someone can prove to me the private lock case, I'd be more willing to bite.
> 
> Thanks for all the OOM context :)

Welcome. The oom handling is a maze and it is really easy to miss all
the subtlety and conflicting requirements that are applied here.
-- 
Michal Hocko
SUSE Labs