Re: [Xen-devel] [PATCH] xen/balloon: set ballooned out pages as invalid in p2m

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 01/07/14 14:58, Konrad Rzeszutek Wilk wrote:
> On Tue, Jul 01, 2014 at 02:37:48PM +0100, David Vrabel wrote:
>> Since cd9151e26d31048b2b5e00fd02e110e07d2200c9 (xen/balloon: set a
>> mapping for ballooned out pages), a ballooned out page had its entry
>> in the p2m set to the MFN of one of the scratch pages.  This means
>> that the p2m will contain many entries pointing to the same MFN.
>>
>> During a domain save, these many-to-one entries are not identified as
>> such and the scratch page is saved multiple times. On restore the
>> ballooned pages are populated with new frames and the domain may use
>> up its allocation before all pages can be restored.
>>
>> Set ballooned out pages as INVALID_P2M_ENTRY in the p2m (as they were
>> before), preventing them from being saved and re-populated on restore.
>>
> 
> Won't that invalide the primal purpose of the scratch page code?
> 
> That is  cd9151e26d31048b2b5e00fd02e110e07d2200c9
> 
>     xen/balloon: set a mapping for ballooned out pages
> " Allocate a page per cpu and map all the ballooned out pages to the
>     corresponding mfn. Set the p2m accordingly. This way reading from a
>     ballooned out page won't cause a kernel crash (see
>     http://lists.xen.org/archives/html/xen-devel/2012-12/msg01154.html).
> "
> ?

No, because we still have a virtual mapping for the ballooned out page.

David

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