linux-kernel - Re: general protection fault in oom_unkillable

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Message-ID: <20190617101310.GB1492@dhcp22.suse.cz>
Date:   Mon, 17 Jun 2019 12:13:10 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Shakeel Butt <shakeelb@...gle.com>,
        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 Mon 17-06-19 18:56:47, Tetsuo Handa wrote:
> On 2019/06/17 15:33, Michal Hocko wrote:
> > On Sat 15-06-19 09:11:37, Shakeel Butt wrote:
> >> On Sat, Jun 15, 2019 at 6:50 AM Michal Hocko <mhocko@...nel.org> wrote:
> > [...]
> >>> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> >>> index 5a58778c91d4..43eb479a5dc7 100644
> >>> --- a/mm/oom_kill.c
> >>> +++ b/mm/oom_kill.c
> >>> @@ -161,8 +161,8 @@ static bool oom_unkillable_task(struct task_struct *p,
> >>>                 return true;
> >>>
> >>>         /* When mem_cgroup_out_of_memory() and p is not member of the group */
> >>> -       if (memcg && !task_in_mem_cgroup(p, memcg))
> >>> -               return true;
> >>> +       if (memcg)
> >>> +               return false;
> >>
> >> This will break the dump_tasks() usage of oom_unkillable_task(). We
> >> can change dump_tasks() to traverse processes like
> >> mem_cgroup_scan_tasks() for memcg OOMs.
> > 
> > Right you are. Doing a similar trick to the oom victim selection is
> > indeed better. We should really strive to not doing a global process
> > iteration when we can do a targeted scan. Care to send a patch?
> 
> I posted a patch that (as a side effect) avoids oom_unkillable_task() from dump_tasks() at
> https://lore.kernel.org/linux-mm/1558519686-16057-2-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp/ .
> What do you think?

I am sorry but I didn't get to look at this series yet. Anyway, changing
the dump_tasks to use a cgroup iterator for the memcg oom sounds like a
straight forward thing to do without making much more changes around.
Global task list iteration under a single RCU is a more complex problem
that is not limited to the OOM path.

-- 
Michal Hocko
SUSE Labs