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From:   Valentin Schneider <valentin.schneider@arm.com>
To:     Woody Lin <woodylin@google.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Ben Segall <bsegall@google.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/scs: Reset the shadow stack when idle_task_exit
In-Reply-To: <CAHn4DedEV953QULZLAPOuXyHAw_mWRTdFj8bSm6zk3rNjFJj-A@mail.gmail.com>
References: <20211012083521.973587-1-woodylin@google.com> <87zgrek1gl.mognet@arm.com> <CAHn4Dec0Jyc30vWMLAXwQ-ge4eS5S26hxfMky-e4f-TTtFrbEQ@mail.gmail.com> <87wnmijysj.mognet@arm.com> <CAHn4DedEV953QULZLAPOuXyHAw_mWRTdFj8bSm6zk3rNjFJj-A@mail.gmail.com>
Date:   Wed, 13 Oct 2021 14:32:51 +0100
Message-ID: <87tuhljbi4.mognet@arm.com>
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On 13/10/21 09:22, Woody Lin wrote:
> On Tue, Oct 12, 2021 at 6:57 PM Valentin Schneider <valentin.schneider@arm.com> wrote:
>> On 12/10/21 18:35, Woody Lin wrote:
>> > unpoison looks more like an one-time thing to me; the idle tasks will
>> > reuse the same stack pages until system resets, so I think we don't need
>> > to re-unpoison that during hotplugging as long as it's unpoisoned in
>> > 'init_idle'.
>> >
>>
>> I would tend to agree, but was bitten by s390 freeing some memory on
>> hot-unplug and re-allocating it upon hotplug:
>>
>>   6a942f578054 ("s390: preempt: Fix preempt_count initialization")
>>
>> This makes me doubt whether we can assert the idle task stack pages are
>> perennial vs hotplug on all architectures.
>
> I made a quick study on memory-hotplug and seems that only memory contains
> nothing other than migratable pages can be unplugged. So process stack
> pages should not be a concern for this, since which is an unmovable
> memory.
>
> However I don't have a chance to work on a system that enables
> memory-hotplug so far, so couldn't verify this assumption further. Guess
> we can create a separate thread to clarify this more.
>

That sounds sensible; I'll try to dig some more into this.

As for the SCS change, someone might argue for placing this elsewhere in
the hotplug path, but that looks fine to me:

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>

> Regards,
> Woody
>
>>
>> >>
>> >> >  }
>> >> >
>> >> > --
>> >> > 2.33.0.882.g93a45727a2-goog