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Date: Sat, 17 Nov 2012 16:17:32 +0100
From: Borislav Petkov <bp@alien8.de>
Subject: Re: [PATCH 2/3] x86,mm: drop TLB flush from ptep_set_access_flags
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Rik van Riel <riel@redhat.com>, Alan Cox <alan@lxorguk.ukuu.org.uk>, Ingo Molnar <mingo@kernel.org>, Andi Kleen <andi@firstfloor.org>, Michel Lespinasse <walken@google.com>, Peter Zijlstra <a.p.zijlstra@chello.nl>, Andrea Arcangeli <aarcange@redhat.com>, Mel Gorman <mgorman@suse.de>, Johannes Weiner <hannes@cmpxchg.org>, Thomas Gleixner <tglx@linutronix.de>, Andrew Morton <akpm@linux-foundation.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>, Florian Fainelli <florian@openwrt.org>, Borislav Petkov <borislav.petkov@amd.com>

On Sat, Nov 17, 2012 at 06:56:10AM -0800, Linus Torvalds wrote:
> I wonder how we could actually test for it. We'd have to have some
> per-cpu page-fault address check (along with a generation count on the
> mm or similar). I doubt we'd figure out anything that works reliably
> and efficiently and would actually show any problems (plus we would
> have no way to ever know we even got the code right, since presumably
> we'd never find hardware that actually shows the behavior we'd be
> looking for..)

Hmm, touching some wrong page through the stale TLB entry could be a
pretty nasty issue to debug. But you're probably right: how does one
test cheaply whether a PTE just got kicked out of the TLB? Maybe mark it
not-present but this would force a rewalk in the case when it is shared,
which is penalty we don't want to pay.

Oh well...

-- 
Regards/Gruss,
Boris.

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