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

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Message-ID: <CAHk-=wjhxzx3KHHOMvdDj3Aw-_Mk5eRiNTUBB=tFf=vTkw1FeA@mail.gmail.com>
Date:   Mon, 21 Sep 2020 09:24:45 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.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 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.

Heh.

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.

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.

             Linus