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

[Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>]

On Tue, Jul 01, 2014 at 03:00:16PM +0100, David Vrabel wrote:
> 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.

If you could add that comment in that would reflect that - IMHO
it would be good.

> 
> David