linux-kernel - Re: Populating multiple ptes at fault time

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Message-ID: <48D2E65A.6020004@redhat.com>
Date:	Thu, 18 Sep 2008 16:38:02 -0700
From:	Avi Kivity <avi@...hat.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
CC:	Nick Piggin <nickpiggin@...oo.com.au>,
	Hugh Dickens <hugh@...itas.com>,
	Linux Memory Management List <linux-mm@...ck.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Avi Kivity <avi@...ranet.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: Populating multiple ptes at fault time

Jeremy Fitzhardinge wrote:
> Avi Kivity wrote:
>   
>>> Do you need to set the A bit synchronously?  
>>>       
>> Yes, of course (if no guest cooperation).
>>     
>
> Is the A bit architecturally guaranteed to be synchronously set?  

I believe so.  The cpu won't cache tlb entries with the A bit clear 
(much like the shadow code), and will rmw the pte on first access.

> Can
> speculative accesses set it?  

Yes, but don't abuse this.

>> If we add an async mode for guests that can cope, maybe this is
>> workable.  I guess this is what you're suggesting.
>>
>>     
>
> Yes.  At worst Linux would underestimate the process RSS a bit
> (depending on how many unsynchronized ptes you leave lying around).  I
>   

Not the RSS (that's pte.present pages) but the working set (aka active 
list).

> bet there's an appropriate pvop hook you could use to force
> synchronization just before the kernel actually inspects the bits
> (leaving lazy mode sounds good).
>   

It would have to be a new lazy mode, not the existing one, I think.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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