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

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Date:   Mon, 21 Sep 2020 21:27:57 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
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Subject: Re: [patch RFC 00/15] mm/highmem: Provide a preemptible variant of kmap_atomic & friends

On Mon, Sep 21 2020 at 09:24, Linus Torvalds wrote:
> On Mon, Sep 21, 2020 at 12:39 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>
>> If a task is migrated to a different CPU then the mapping address will
>> change which will explode in colourful ways.
>
> Right you are.
>
> Maybe we really *could* call this new kmap functionality something
> like "kmap_percpu()" (or maybe "local" is good enough), and make it
> act like your RT code does for spinlocks - not disable preemption, but
> only disabling CPU migration.

I"m all for it, but the scheduler people have opinions :)

> That would probably be good enough for a lot of users that don't want
> to expose excessive latencies, but where it's really not a huge deal
> to say "stick to this CPU for a short while".
>
> The crypto code certainly sounds like one such case.

I looked at a lot of the kmap_atomic() places and quite some of them
only require migration to be disabled to keep the temporary map
stable.

Quite some code could be simplified significantly especially those
places which need to do copy_from/to_user inside these
sections. Graphics is the main example here as Daniel pointed out.

Alternatively this could of course be solved with per CPU page tables
which will come around some day anyway I fear.

Thanks,

        tglx