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Message-ID: <CAMj1kXHbKVY_3s_DX_iv0gDbM+mcnP2Eh9ZkeXMPA9sZQVvapw@mail.gmail.com>
Date:   Thu, 17 Sep 2020 09:38:54 +0300
From:   Ard Biesheuvel <ardb@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Paul E. McKenney" <paulmck@...nel.org>,
        Daniel Vetter <daniel@...ll.ch>,
        Thomas Gleixner <tglx@...utronix.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        LKML <linux-kernel@...r.kernel.org>,
        linux-arch <linux-arch@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Valentin Schneider <valentin.schneider@....com>,
        Richard Henderson <rth@...ddle.net>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        alpha <linux-alpha@...r.kernel.org>,
        Jeff Dike <jdike@...toit.com>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        linux-um <linux-um@...ts.infradead.org>,
        Brian Cain <bcain@...eaurora.org>,
        linux-hexagon@...r.kernel.org,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        Ingo Molnar <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
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        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Will Deacon <will@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux-MM <linux-mm@...ck.org>, Ingo Molnar <mingo@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        linux-xtensa@...ux-xtensa.org,
        Jani Nikula <jani.nikula@...ux.intel.com>,
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        dri-devel <dri-devel@...ts.freedesktop.org>,
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        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Shuah Khan <shuah@...nel.org>, rcu@...r.kernel.org,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>
Subject: Re: [patch 00/13] preempt: Make preempt count unconditional

On Wed, 16 Sep 2020 at 21:32, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
>
> But something like a driver list walking thing should not be doing
> different things behind peoples back depending on whether they hold
> spinlocks or not. It should either just work regardless, or there
> should be a flag (or special interface) for the "you're being called
> in a crtitical region".
>
> Because dynamically changing behavior really is very confusing.
>

By the same reasoning, I don't think a generic crypto library should
be playing tricks with preemption en/disabling under the hood when
iterating over some data that is all directly accessible via the
linear map on the platforms that most people care about. And using
kmap_atomic() unconditionally achieves exactly that.

As I argued before, the fact that kmap_atomic() can be called from an
atomic context, and the fact that its implementation on HIGHMEM
platforms requires preemption to be disabled until the next kunmap()
are two different things, and I don't agree with your assertion that
the name kmap_atomic() implies the latter semantics. If we can avoid
disabling preemption on HIGHMEM, as Thomas suggests, we surely don't
need it on !HIGHMEM either, and given that kmap_atomic() is preferred
today anyway, we can just merge the two implementations. Are there any
existing debug features that could help us spot [ab]use of things like
raw per-CPU data within kmap_atomic regions?

Re your point about deprecating HIGHMEM: some work is underway on ARM
to implement a 3.75/3.75 GB kernel/user split on recent LPAE capable
hardware (which shouldn't suffer from the performance issues that
plagued the 4/4 split on i686), and so hopefully, there is a path
forward for ARM that does not rely on HIGHMEM as it does today.

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