xen/balloon: Fix mapping PG_offline pages to user space

The XEN balloon driver - in contrast to other balloon drivers - allows
to map some inflated pages to user space. Such pages are allocated via
alloc_xenballooned_pages() and freed via free_xenballooned_pages().
The pfn space of these allocated pages is used to map other things
by the hypervisor using hypercalls.

Pages marked with PG_offline must never be mapped to user space (as
this page type uses the mapcount field of struct pages).

So what we can do is, clear/set PG_offline when allocating/freeing an
inflated pages. This way, most inflated pages can be excluded by
dumping tools and the "reused for other purpose" balloon pages are
correctly not marked as PG_offline.

Fixes: 77c4adf6 (xen/balloon: mark inflated pages PG_offline)
Reported-by: Julien Grall <julien.grall@arm.com>
Tested-by: Julien Grall <julien.grall@arm.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/balloon.c

@@ -604,6 +604,7 @@ int alloc_xenballooned_pages(int nr_pages, struct page **pages)
 	while (pgno < nr_pages) {
 		page = balloon_retrieve(true);
 		if (page) {
+			__ClearPageOffline(page);
 			pages[pgno++] = page;
 #ifdef CONFIG_XEN_HAVE_PVMMU
 			/*
@@ -645,9 +646,11 @@ void free_xenballooned_pages(int nr_pages, struct page **pages)
 	mutex_lock(&balloon_mutex);
 
 	for (i = 0; i < nr_pages; i++) {
-		if (pages[i])
+		if (pages[i]) {
+			__SetPageOffline(pages[i]);
 			balloon_append(pages[i]);
 		}
+	}
 
 	balloon_stats.target_unpopulated -= nr_pages;