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Date:   Fri, 8 Apr 2022 06:36:40 -0400
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Subject: Re: [PATCH v8] oom_kill.c: futex: Don't OOM reap the VMA containing
 the robust_list_head
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To:     Michal Hocko <mhocko@suse.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Rafael Aquini <aquini@redhat.com>,
        Waiman Long <longman@redhat.com>, Baoquan He <bhe@redhat.com>,
        Christoph von Recklinghausen <crecklin@redhat.com>,
        Don Dutile <ddutile@redhat.com>,
        "Herton R . Krzesinski" <herton@redhat.com>,
        David Rientjes <rientjes@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Davidlohr Bueso <dave@stgolabs.net>,
        Ingo Molnar <mingo@redhat.com>,
        Joel Savitz <jsavitz@redhat.com>,
        Darren Hart <dvhart@infradead.org>, stable@kernel.org
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From:   Nico Pache <npache@redhat.com>
In-Reply-To: <YlAHfU7VEI0wYAb/@dhcp22.suse.cz>
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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