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

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Message-ID: <4638E43F.6000806@vmware.com>
Date:	Wed, 02 May 2007 12:19:27 -0700
From:	Zachary Amsden <zach@...are.com>
To:	Andi Kleen <ak@...ell.com>
CC:	Jan Beulich <jbeulich@...ell.com>, linux-kernel@...r.kernel.org,
	patches@...-64.org
Subject: Re: [patches] [PATCH] [28/34] i386: pte xchg optimization

Andi Kleen wrote:
>> 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.
>>     
>
> Yes I agree. Might be a nice speed up for exit to distingush the cases.
>   

That is what I did in this patch - but with care zap_pte_range can also 
be called in non-teardown cases when unmapping (when !fullmm) and user 
mappings could still be in effect on remote processors.

Zach
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