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

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Date:   Fri, 14 Jan 2022 09:58:35 -0500
From:   Waiman Long <longman@...hat.com>
To:     Joel Savitz <jsavitz@...hat.com>, Michal Hocko <mhocko@...e.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, 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


On 1/14/22 09:55, Waiman Long wrote:
> On 1/14/22 09:39, 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.
>>
>> Since in the case of an oom victim using robust futexes the core
>> assumption of the oom reaper is violated, we propose to solve this
>> problem by either canceling or delaying the waking of the oom reaper
>> thread by wake_oom_reaper in the case that tsk->robust_list is
>> non-NULL.
>>
>> e.g. the bug does not reproduce with this patch (from 
>> npache@...hat.com):
>>
>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
>> index 989f35a2bbb1..b8c518fdcf4d 100644
>> --- a/mm/oom_kill.c
>> +++ b/mm/oom_kill.c
>> @@ -665,6 +665,19 @@ static void wake_oom_reaper(struct task_struct 
>> *tsk)
>>          if (test_and_set_bit(MMF_OOM_REAP_QUEUED, 
>> &tsk->signal->oom_mm->flags))
>>                  return;
>>
>> +#ifdef CONFIG_FUTEX
>> +       /*
>> +        * don't wake the oom_reaper thread if we still have a robust
>> list to handle
>> +        * This will then rely on the sigkill to handle the cleanup 
>> of memory
>> +        */
>> +       if(tsk->robust_list)
>> +               return;
>> +#ifdef CONFIG_COMPAT
>> +       if(tsk->compat_robust_list)
>> +               return;
>> +#endif
>> +#endif
>> +
>>          get_task_struct(tsk);
>>
>>          spin_lock(&oom_reaper_lock);
>
> OK, that can explain why the robust futex is not properly cleaned up. 
> Could you post a more formal v2 patch with description about the 
> possible race condition?
>
It should be v3. Sorry for the mix-up.

Cheers,
Longman