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

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.

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 = 
                        if (!PageHighMem(page)) {
+                               struct page *scratch_page = 
                                ret = HYPERVISOR_update_va_mapping(
                                                (unsigned long)__va(pfn << 
                                                        PAGE_KERNEL_RO), 0);
-                       }
-                       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);

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