linux-kernel - Re: [PATCH 2/3] x86,mm: drop TLB flush from ptep_set_access

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Date:	Sat, 17 Nov 2012 16:17:32 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Rik van Riel <riel@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <andi@...stfloor.org>,
	Michel Lespinasse <walken@...gle.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>,
	Florian Fainelli <florian@...nwrt.org>,
	Borislav Petkov <borislav.petkov@....com>
Subject: Re: [PATCH 2/3] x86,mm: drop TLB flush from ptep_set_access_flags

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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