linux-kernel - Re: [PATCH] proc, oom: do not report alien mms when setting oom_score

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Message-ID: <46b3262e-2a9a-da01-16de-14cd4d7eaa40@i-love.sakura.ne.jp>
Date:   Wed, 13 Feb 2019 06:07:26 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Michal Hocko <mhocko@...nel.org>
Cc:     David Rientjes <rientjes@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Yong-Taek Lee <ytk.lee@...sung.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [PATCH] proc, oom: do not report alien mms when setting
 oom_score_adj

On 2019/02/13 5:56, Andrew Morton wrote:
> On Tue, 12 Feb 2019 11:21:29 +0100 Michal Hocko <mhocko@...nel.org> wrote:
> 
>> Tetsuo has reported that creating a thousands of processes sharing MM
>> without SIGHAND (aka alien threads) and setting
>> /proc/<pid>/oom_score_adj will swamp the kernel log and takes ages [1]
>> to finish. This is especially worrisome that all that printing is done
>> under RCU lock and this can potentially trigger RCU stall or softlockup
>> detector.
>>
>> The primary reason for the printk was to catch potential users who might
>> depend on the behavior prior to 44a70adec910 ("mm, oom_adj: make sure
>> processes sharing mm have same view of oom_score_adj") but after more
>> than 2 years without a single report I guess it is safe to simply remove
>> the printk altogether.
>>
>> The next step should be moving oom_score_adj over to the mm struct and
>> remove all the tasks crawling as suggested by [2]
>>
>> [1] http://lkml.kernel.org/r/97fce864-6f75-bca5-14bc-12c9f890e740@i-love.sakura.ne.jp
>> [2] http://lkml.kernel.org/r/20190117155159.GA4087@dhcp22.suse.cz
> 
> I think I'll put a cc:stable on this.  Deleting a might-trigger debug
> printk is safe and welcome.
> 

Putting cc:stable is fine. But I doubt the usefulness of this patch.
If nobody really depends on the behavior prior to 44a70adec910,
we should remove the pointless (otherwise racy) iteration itself.