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From:   Zhaoyang Huang <huangzhaoyang@gmail.com>
Date:   Tue, 10 Apr 2018 16:04:40 +0800
Message-ID: <CAGWkznHQSZ5jPF2njOp0BritTmKiGOToq5bAz1bhzzmFi_01nA@mail.gmail.com>
Subject: Re: [PATCH v1] ringbuffer: Don't choose the process with adj equal OOM_SCORE_ADJ_MIN
To:     Michal Hocko <mhocko@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Apr 10, 2018 at 3:49 PM, Michal Hocko <mhocko@kernel.org> wrote:
> On Tue 10-04-18 14:39:35, Zhaoyang Huang wrote:
>> On Tue, Apr 10, 2018 at 2:14 PM, Michal Hocko <mhocko@kernel.org> wrote:
>> > On Tue 10-04-18 11:41:44, Zhaoyang Huang wrote:
>> >> On Tue, Apr 10, 2018 at 11:12 AM, Steven Rostedt <rostedt@goodmis.org> wrote:
>> >> > On Tue, 10 Apr 2018 10:32:36 +0800
>> >> > Zhaoyang Huang <huangzhaoyang@gmail.com> wrote:
>> >> >
>> >> >> For bellowing scenario, process A have no intension to exhaust the
>> >> >> memory, but will be likely to be selected by OOM for we set
>> >> >> OOM_CORE_ADJ_MIN for it.
>> >> >> process A(-1000)                                          process B
>> >> >>
>> >> >>   i = si_mem_available();
>> >> >>        if (i < nr_pages)
>> >> >>            return -ENOMEM;
>> >> >>                                                    schedule
>> >> >>                                                 --------------->
>> >> >> allocate huge memory
>> >> >>                                                 <-------------
>> >> >> if (user_thread)
>> >> >>   set_current_oom_origin();
>> >> >>
>> >> >>   for (i = 0; i < nr_pages; i++) {
>> >> >>          bpage = kzalloc_node
>> >> >
>> >> > Is this really an issue though?
>> >> >
>> >> > Seriously, do you think you will ever hit this?
>> >> >
>> >> > How often do you increase the size of the ftrace ring buffer? For this
>> >> > to be an issue, the system has to trigger an OOM at the exact moment
>> >> > you decide to increase the size of the ring buffer. That would be an
>> >> > impressive attack, with little to gain.
>> >> >
>> >> > Ask the memory management people. If they think this could be a
>> >> > problem, then I'll be happy to take your patch.
>> >> >
>> >> > -- Steve
>> >> add Michael for review.
>> >> Hi Michael,
>> >> I would like suggest Steve NOT to set OOM_CORE_ADJ_MIN for the process
>> >> with adj = -1000 when setting the user space process as potential
>> >> victim of OOM.
>> >
>> > OOM_SCORE_ADJ_MIN means "hide the process from the OOM killer completely".
>> > So what exactly do you want to achieve here? Because from the above it
>> > sounds like opposite things. /me confused...
>> >
>> Steve's patch intend to have the process be OOM's victim when it
>> over-allocating pages for ring buffer. I amend a patch over to protect
>> process with OOM_SCORE_ADJ_MIN from doing so. Because it will make
>> such process to be selected by current OOM's way of
>> selecting.(consider OOM_FLAG_ORIGIN first before the adj)
>
> I just wouldn't really care unless there is an existing and reasonable
> usecase for an application which updates the ring buffer size _and_ it
> is OOM disabled at the same time.
> --
> Michal Hocko
> SUSE Labs
There is indeed such kind of test case on my android system, which is
known as CTS and Monkey etc. Furthermore, I think we should make the
patch to be as safest as possible. Why do we leave a potential risk
here? There is no side effect for my patch.