linux-kernel - Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap

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Message-ID: <87a6xjd1dw.fsf@nanos.tec.linutronix.de>
Date:   Mon, 21 Sep 2020 09:39:39 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

On Sun, Sep 20 2020 at 10:42, Linus Torvalds wrote:
> On Sun, Sep 20, 2020 at 10:40 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> I think the more obvious solution is to split the whole exercise:
>>
>>   schedule()
>>      prepare_switch()
>>         unmap()
>>
>>     switch_to()
>>
>>     finish_switch()
>>         map()
>
> Yeah, that looks much easier to explain. Ack.

So far so good, but Peter Z. just pointed out to me that I completely
missed the fact that this cannot work.

If a task is migrated to a different CPU then the mapping address will
change which will explode in colourful ways.

On RT kernels this works because we ping the task to the CPU via
migrate_disable(). On a !RT kernel migrate_disable() maps to
preempt_disable() which brings us back to square one.

/me goes back to the drawing board.

Thanks,

        tglx