linux-kernel - Re: [patch v4] mm, oom: fix unnecessary killing of additional processes

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <05dbc69a-1c26-adec-15c6-f7192f8d2ae0@i-love.sakura.ne.jp>
Date:   Wed, 25 Jul 2018 07:31:24 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...e.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch v4] mm, oom: fix unnecessary killing of additional
 processes

On 2018/07/25 6:45, David Rientjes wrote:
> On Sat, 21 Jul 2018, Tetsuo Handa wrote:
> 
>> You can't apply "[patch v4] mm, oom: fix unnecessary killing of additional processes"
>> because Michal's patch which removes oom_lock serialization was added to -mm tree.
>>
> 
> I've rebased the patch to linux-next and posted a v5.
> 
>> You might worry about situations where __oom_reap_task_mm() is a no-op.
>> But that is not always true. There is no point with emitting
>>
>>   pr_info("oom_reaper: unable to reap pid:%d (%s)\n", ...);
>>   debug_show_all_locks();
>>
>> noise and doing
>>
>>   set_bit(MMF_OOM_SKIP, &mm->flags);
>>
>> because exit_mmap() will not release oom_lock until __oom_reap_task_mm()
>> completes. That is, except extra noise, there is no difference with
>> current behavior which sets set_bit(MMF_OOM_SKIP, &mm->flags) after
>> returning from __oom_reap_task_mm().
>>
> 
> v5 has restructured how exit_mmap() serializes its unmapping with the oom 
> reaper.  It sets MMF_OOM_SKIP while holding mm->mmap_sem.
> 

I think that v5 is still wrong. exit_mmap() keeps mmap_sem held for write does
not prevent oom_reap_task() from emitting the noise and setting MMF_OOM_SKIP
after timeout. Since your purpose is to wait for release of memory which could
not be reclaimed by __oom_reap_task_mm(), what if __oom_reap_task_mm() was no-op and
exit_mmap() was preempted immediately after returning from __oom_reap_task_mm() ?

Also, I believe that userspace visible knob is not needed.