linux-kernel - Re: general protection fault in oom_unkillable

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Message-ID: <CALvZod5L9VEAiGSk2JYY-e7RGLRn+tFcn-cePtw-epLGsxf2wg@mail.gmail.com>
Date:   Mon, 17 Jun 2019 21:21:20 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Michal Hocko <mhocko@...nel.org>,
        syzbot <syzbot+d0fc9d3c166bc5e4a94b@...kaller.appspotmail.com>,
        "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, Jun 17, 2019 at 6:45 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Mon, 17 Jun 2019 06:23:07 -0700 Shakeel Butt <shakeelb@...gle.com> wrote:
>
> > > Here is a patch to use CSS_TASK_ITER_PROCS.
> > >
> > > From 415e52cf55bc4ad931e4f005421b827f0b02693d Mon Sep 17 00:00:00 2001
> > > From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> > > Date: Mon, 17 Jun 2019 00:09:38 +0900
> > > Subject: [PATCH] mm: memcontrol: Use CSS_TASK_ITER_PROCS at mem_cgroup_scan_tasks().
> > >
> > > Since commit c03cd7738a83b137 ("cgroup: Include dying leaders with live
> > > threads in PROCS iterations") corrected how CSS_TASK_ITER_PROCS works,
> > > mem_cgroup_scan_tasks() can use CSS_TASK_ITER_PROCS in order to check
> > > only one thread from each thread group.
> > >
> > > Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> >
> > Reviewed-by: Shakeel Butt <shakeelb@...gle.com>
> >
> > Why not add the reproducer in the commit message?
>
> That would be nice.
>
> More nice would be, as always, a descriptoin of the user-visible impact
> of the patch.
>

This is just a cleanup and optimization where instead of traversing
all the threads in a memcg, we only traverse only one thread for each
thread group in a memcg. There is no user visible impact.

> As I understand it, it's just a bit of a cleanup against current
> mainline but without this patch in place, Shakeel's "mm, oom: refactor
> dump_tasks for memcg OOMs" will cause kernel crashes.  Correct?

No, the patch "mm, oom: refactor dump_tasks for memcg OOMs" is making
dump_stacks not depend on the memcg check within
oom_unkillable_task().

"mm, oom: fix oom_unkillable_task for memcg OOMs" is the actual fix
which is making oom_unkillable_task() correctly handle the memcg OOMs
code paths.

Shakeel