xen/grant-table: Use put_page instead of free_page

The page given to gnttab_end_foreign_access() to free could be a
compound page so use put_page() instead of free_page() since it can
handle both compound and single pages correctly.

This bug was discovered when migrating a Xen VM with several VIFs and
CONFIG_DEBUG_VM enabled. It hits a BUG usually after fewer than 10
iterations. All netfront devices disconnect from the backend during a
suspend/resume and this will call gnttab_end_foreign_access() if a
netfront queue has an outstanding skb. The mismatch between calling
get_page() and free_page() on a compound page causes a reference
counting error which is detected when DEBUG_VM is enabled.
Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/grant-table.c

@@ -382,7 +382,7 @@ static void gnttab_handle_deferred(struct timer_list *unused)
 			if (entry->page) {
 				pr_debug("freeing g.e. %#x (pfn %#lx)\n",
 					 entry->ref, page_to_pfn(entry->page));
-				__free_page(entry->page);
+				put_page(entry->page);
 			} else
 				pr_info("freeing g.e. %#x\n", entry->ref);
 			kfree(entry);
@@ -438,7 +438,7 @@ void gnttab_end_foreign_access(grant_ref_t ref, int readonly,
 	if (gnttab_end_foreign_access_ref(ref, readonly)) {
 		put_free_entry(ref);
 		if (page != 0)
-			free_page(page);
+			put_page(virt_to_page(page));
 	} else
 		gnttab_add_deferred(ref, readonly,
 				    page ? virt_to_page(page) : NULL);