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

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

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).
"
?

> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> Reported-by: Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> Tested-by: Marek Marczykowski <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> ---
>  drivers/xen/balloon.c |   12 +++++-------
>  1 file changed, 5 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> index b7a506f..5c660c7 100644
> --- a/drivers/xen/balloon.c
> +++ b/drivers/xen/balloon.c
> @@ -426,20 +426,18 @@ static enum bp_state decrease_reservation(unsigned long 
> nr_pages, gfp_t gfp)
>                * p2m are consistent.
>                */
>               if (!xen_feature(XENFEAT_auto_translated_physmap)) {
> -                     unsigned long p;
> -                     struct page   *scratch_page = 
> get_balloon_scratch_page();
> -
>                       if (!PageHighMem(page)) {
> +                             struct page *scratch_page = 
> get_balloon_scratch_page();
> +
>                               ret = HYPERVISOR_update_va_mapping(
>                                               (unsigned long)__va(pfn << 
> PAGE_SHIFT),
>                                               
> pfn_pte(page_to_pfn(scratch_page),
>                                                       PAGE_KERNEL_RO), 0);
>                               BUG_ON(ret);
> -                     }
> -                     p = page_to_pfn(scratch_page);
> -                     __set_phys_to_machine(pfn, pfn_to_mfn(p));
>  
> -                     put_balloon_scratch_page();
> +                             put_balloon_scratch_page();
> +                     }
> +                     __set_phys_to_machine(pfn, INVALID_P2M_ENTRY);
>               }
>  #endif
>  
> -- 
> 1.7.10.4
> 

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