linux-kernel - Re: [patch 0/6] Guest page hinting version 6.

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Date:	Fri, 14 Mar 2008 11:30:13 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Hugh Dickins <hugh@...itas.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
	virtualization@...ts.osdl.org, akpm@...ux-foundation.org,
	nickpiggin@...oo.com.au, frankeh@...son.ibm.com,
	rusty@...tcorp.com.au, andrea@...ranet.com, clameter@....com,
	a.p.zijlstra@...llo.nl, Keir Fraser <keir@...source.com>,
	Ian Pratt <ian.pratt@...source.com>
Subject: Re: [patch 0/6] Guest page hinting version 6.

Zachary Amsden wrote:
> On Thu, 2008-03-13 at 18:55 +0000, Hugh Dickins wrote:
>   
>> On Thu, 13 Mar 2008, Jeremy Fitzhardinge wrote:
>>     
>>> My other concern is just correctness over time on the Linux side.  We already
>>> have enough trouble keeping things like the pte and page structure state in
>>> sync, with resulting rare data-loss bugs.  Adding another layer which only
>>> applies in specific environments raises the possibility for new bugs to be
>>> un-noticed for a long time.  How can we structure the VM changes to make sure
>>> that its robust in the face of maintenance?
>>>       
>> Yes, that's the main concern, as whenever lots of subtlety is added.
>> I wonder if there's any chance of a CONFIG_DEBUG mode, which could be
>> run on anybody's x86 machine, without involving any virtualization, but
>> in which the PAGE_STATEs become essential to the correct working of the mm.
>>     
>
> How about a fake hypervisor, which is really just a random page evictor,
> following the rules of CMM?
>   

Probably simpler to just have variants of the page_set_* functions which 
simulate the worst-possible host action immediately (ie, stealing pages, 
logically swapping them, etc).  That wouldn't give you full coverage, 
but it would go some way.  An async variant which schedules a change in 
a few milliseconds would help too.

I guess that's equivalent to having a special-purpose hypervisor built 
into the kernel (hm, sounds familiar...).

    J
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