linux-kernel - Re: [RFC PATCH] memcg, oom: throttle dump_header for memcg ooms without eligible tasks

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Message-ID: <6c0a57b3-bfd4-d832-b0bd-5dd3bcae460e@i-love.sakura.ne.jp>
Date:   Mon, 15 Oct 2018 21:47:08 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>, linux-mm@...ck.org,
        syzkaller-bugs@...glegroups.com, guro@...com,
        kirill.shutemov@...ux.intel.com, linux-kernel@...r.kernel.org,
        rientjes@...gle.com, yang.s@...baba-inc.com,
        Andrew Morton <akpm@...ux-foundation.org>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH] memcg, oom: throttle dump_header for memcg ooms
 without eligible tasks

On 2018/10/15 20:24, Michal Hocko wrote:
> On Mon 15-10-18 19:57:35, Tetsuo Handa wrote:
>> On 2018/10/15 17:19, Michal Hocko wrote:
>>> As so many dozens of times before, I will point you to an incremental
>>> nature of changes we really prefer in the mm land. We are also after a
>>> simplicity which your proposal lacks in many aspects. You seem to ignore
>>> that general approach and I have hard time to consider your NAK as a
>>> relevant feedback. Going to an extreme and basing a complex solution on
>>> it is not going to fly. No killable process should be a rare event which
>>> requires a seriously misconfigured memcg to happen so wildly. If you can
>>> trigger it with a normal user privileges then it would be a clear bug to
>>> address rather than work around with printk throttling.
>>>
>>
>> I can trigger 200+ times / 900+ lines / 69KB+ of needless OOM messages
>> with a normal user privileges. This is a lot of needless noise/delay.
> 
> I am pretty sure you have understood the part of my message you have
> chosen to not quote where I have said that the specific rate limitting
> decisions can be changed based on reasonable configurations. There is
> absolutely zero reason to NAK a natural decision to unify the throttling
> and cook a per-memcg way for a very specific path instead.
> 
>> No killable process is not a rare event, even without root privileges.
>>
>> [root@...ecurity kumaneko]# time ./a.out
>> Killed
>>
>> real    0m2.396s
>> user    0m0.000s
>> sys     0m2.970s
>> [root@...ecurity ~]# dmesg | grep 'no killable' | wc -l
>> 202
>> [root@...ecurity ~]# dmesg | wc
>>     942    7335   70716
> 
> OK, so this is 70kB worth of data pushed throug the console. Is this
> really killing any machine?
> 

Nobody can prove that it never kills some machine. This is just one example result of
one example stress tried in my environment. Since I am secure programming man from security
subsystem, I really hate your "Can you trigger it?" resistance. Since this is OOM path
where nobody tests, starting from being prepared for the worst case keeps things simple.