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

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Message-ID: <87h7rscqe6.fsf@nanos.tec.linutronix.de>
Date:   Sun, 20 Sep 2020 19:24:49 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Daniel Vetter <daniel@...ll.ch>
Cc:     Daniel Vetter <daniel@...ll.ch>,
        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 10:23, Daniel Vetter wrote:

> On Sun, Sep 20, 2020 at 08:23:26AM +0200, Thomas Gleixner wrote:
>> On Sat, Sep 19 2020 at 12:37, Daniel Vetter wrote:
>> > On Sat, Sep 19, 2020 at 12:35 PM Daniel Vetter <daniel@...ll.ch> wrote:
>> >> I think it should be the case, but I want to double check: Will
>> >> copy_*_user be allowed within a kmap_temporary section? This would
>> >> allow us to ditch an absolute pile of slowpaths.
>> >
>> > (coffee just kicked in) copy_*_user is ofc allowed, but if you hit a
>> > page fault you get a short read/write. This looks like it would remove
>> > the need to handle these in a slowpath, since page faults can now be
>> > served in this new kmap_temporary sections. But this sounds too good
>> > to be true, so I'm wondering what I'm missing.
>> 
>> In principle we could allow pagefaults, but not with the currently
>> proposed interface which can be called from any context. Obviously if
>> called from atomic context it can't handle user page faults.
>  
> Yeah that's clear, but does the implemention need to disable pagefaults
> unconditionally?

As I wrote in the other reply it's not required and the final interface
will neither disable preemption nor pagefaults (except for the atomic
wrapper around it).

Thanks,

        tglx