xen/balloon: Avoid OOM when requesting highmem

If highmem pages are requested from the balloon on a system without
highmem, the implementation of alloc_xenballooned_pages will allocate
all available memory trying to find highmem pages to return. Allow
low memory to be returned when highmem pages are requested to avoid
this loop.
Signed-off-by: Daniel De Graaf <dgdegra@tycho.nsa.gov>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/balloon.c

@@ -501,7 +501,7 @@ EXPORT_SYMBOL_GPL(balloon_set_new_target);
  * alloc_xenballooned_pages - get pages that have been ballooned out
  * @nr_pages: Number of pages to get
  * @pages: pages returned
- * @highmem: highmem or lowmem pages
+ * @highmem: allow highmem pages
  * @return 0 on success, error otherwise
  */
 int alloc_xenballooned_pages(int nr_pages, struct page **pages, bool highmem)
@@ -511,7 +511,7 @@ int alloc_xenballooned_pages(int nr_pages, struct page **pages, bool highmem)
 	mutex_lock(&balloon_mutex);
 	while (pgno < nr_pages) {
 		page = balloon_retrieve(highmem);
-		if (page && PageHighMem(page) == highmem) {
+		if (page && (highmem || !PageHighMem(page))) {
 			pages[pgno++] = page;
 		} else {
 			enum bp_state st;