Re: [patch 1/9] Guest page hinting: unused / free pages.

[Hubertus Franke <frankeh@watson.ibm.com>]

Martin Schwidefsky wrote:
> 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.
> 

Interesting..
But don't we have to do some locking on the page to avoid race conditions?
A page needs to be consistent between check through __page_discardable and
committing to the hypervisor. We could raise the PG_state_change flag for
that period.