linux-kernel - Re: general protection fault in oom_unkillable

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Date:   Sun, 16 Jun 2019 06:33:51 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Michal Hocko <mhocko@...nel.org>,
        syzbot <syzbot+d0fc9d3c166bc5e4a94b@...kaller.appspotmail.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Roman Gushchin <guro@...com>,
        Johannes Weiner <hannes@...xchg.org>,
        Jérôme Glisse <jglisse@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        yuzhoujian@...ichuxing.com
Subject: Re: general protection fault in oom_unkillable_task

On 2019/06/16 3:50, Shakeel Butt wrote:
>> While dump_tasks() traverses only each thread group, mem_cgroup_scan_tasks()
>> traverses each thread.
> 
> I think mem_cgroup_scan_tasks() traversing threads is not intentional
> and css_task_iter_start in it should use CSS_TASK_ITER_PROCS as the
> oom killer only cares about the processes or more specifically
> mm_struct (though two different thread groups can have same mm_struct
> but that is fine).

We can't use CSS_TASK_ITER_PROCS from mem_cgroup_scan_tasks(). I've tried
CSS_TASK_ITER_PROCS in an attempt to evaluate only one thread from each
thread group, but I found that CSS_TASK_ITER_PROCS causes skipping whole
threads in a thread group (and trivially allowing "Out of memory and no
killable processes...\n" flood) if thread group leader has already exited.

If we can agree with using a flag in mm_struct in order to track whether 
each mm_struct was already evaluated for each out_of_memory() call, we can
printk() only one thread from all thread groups sharing that mm_struct...