Subject: Re: [patch 1/9] Guest page hinting: unused / free pages.
From: Martin Schwidefsky <schwidefsky@de.ibm.com>
Reply-To: schwidefsky@de.ibm.com
To: Rik van Riel <riel@redhat.com>
Cc: Hubertus Franke <frankeh@watson.ibm.com>, linux-kernel@vger.kernel.org,
       virtualization@lists.osdl.org, akpm@osdl.org, nickpiggin@yahoo.com.au,
       rhim@cc.gatech.edu
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Organization: IBM Corporation
Date: Wed, 13 Sep 2006 16:59:03 +0200
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On Wed, 2006-09-13 at 10:45 -0400, Rik van Riel wrote:
> > But another trouble you have not mentioned is what happens to pages
> > with pending make-volatile that need to and/or have been made stable
> > in the meantime. They too need to be removed from this pending list.
> 
> At the time where you walk the set of pages (pagevec?) to make
> volatile, you can check whether the page flags are still right.

A make volatile can be done anytime as long as the page is in page
cache. Before a page can be made stable the caller needs to make sure
that one of the conditions that prevent a make volatile becomes true.
So a page in the pending make-volatile array does not have to be removed
because a make stable has been done. It only has to be removed if it
gets freed.

> A page that was set to be marked volatile with the hypervisor,
> but later turned stable again would have that indicated in its
> page flags, right?

Several page flag bits and some other conditions like "has a mapping"
and "reference count is map count + 1". Most of the checks that need to
be done for make volatile are on page flags.

-- 
blue skies,
  Martin.

Martin Schwidefsky
Linux for zSeries Development & Services
IBM Deutschland Entwicklung GmbH

"Reality continues to ruin my life." - Calvin.


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