linux-kernel - Re: [patches] [PATCH] [28/34] i386: pte xchg optimization

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Message-Id: <463889A9.76E4.0078.0@novell.com>
Date:	Wed, 02 May 2007 11:52:57 +0100
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Andi Kleen" <ak@...e.de>, <zach@...are.com>
Cc:	<linux-kernel@...r.kernel.org>, <patches@...-64.org>
Subject: Re: [patches] [PATCH] [28/34] i386: pte xchg optimization

>>> Andi Kleen <ak@...e.de> 30.04.07 17:50 >>>
>
>From: Zachary Amsden <zach@...are.com>
>
>In situations where page table updates need only be made locally, and there is
>no cross-processor A/D bit races involved, we need not use the heavyweight
>xchg instruction to atomically fetch and clear page table entries.  Instead,
>we can just read and clear them directly.

I always wondered why the xchg is necessary here at all. If the process of
tearing down a page table entry has started, other users of the mapped
linear address are broken anyway - why is it necessary to still monitor the
effect they may have on the A/D bits, unless this is a transient tear down?

Checking for the uses of ptep_get_and_clear, I would judge that the use in
change_pte_range() may in fact need the xchg, but the uses in
vunmap_pte_range() and zap_pte_range() shouldn't.

Jan

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