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

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Date:   Fri, 8 Apr 2022 06:36:40 -0400
From:   Nico Pache <npache@...hat.com>
To:     Michal Hocko <mhocko@...e.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 4/8/22 05:59, Michal Hocko wrote:
> On Fri 08-04-22 05:40:09, Nico Pache wrote:
>>
>>
>> On 4/8/22 05:36, Michal Hocko wrote:
>>> On Fri 08-04-22 04:52:33, Nico Pache wrote:
>>> [...]
>>>> In a heavily contended CPU with high memory pressure the delay may also
>>>> lead to other processes unnecessarily OOMing.
>>>
>>> Let me just comment on this part because there is likely a confusion
>>> inlved. Delaying the oom_reaper _cannot_ lead to additional OOM killing
>>> because the the oom killing is throttled by existence of a preexisting
>>> OOM victim. In other words as long as there is an alive victim no
>>> further victims are not selected and the oom killer backs off. The
>>> oom_repaer will hide the alive oom victim after it is processed.
>>> The longer the delay will be the longer an oom victim can block a
>>> further progress but it cannot really cause unnecessary OOMing.
>> Is it not the case that if we delay an OOM, the amount of available memory stays
>> limited and other processes that are allocating memory can become OOM candidates?
> 
> No. Have a look at oom_evaluate_task (tsk_is_oom_victim check).
Ok I see.

Doesnt the delay then allow the system to run into the following case more easily?:
pr_warn("Out of memory and no killable processes...\n");
panic("System is deadlocked on memory\n");

If the system cant select another OOM candidate, the oom_reaper is delayed, and
the exit is blocked, then we panic.

-- Nico