xen: add an "highmem" parameter to alloc_xenballooned_pages

Add an highmem parameter to alloc_xenballooned_pages, to allow callers to
request lowmem or highmem pages.

Fix the code style of free_xenballooned_pages' prototype.
Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

drivers/xen/balloon.c

@@ -501,20 +501,24 @@ 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
  * @return 0 on success, error otherwise
  */
-int alloc_xenballooned_pages(int nr_pages, struct page** pages)
+int alloc_xenballooned_pages(int nr_pages, struct page **pages, bool highmem)
 {
 	int pgno = 0;
 	struct page* page;
 	mutex_lock(&balloon_mutex);
 	while (pgno < nr_pages) {
-		page = balloon_retrieve(true);
-		if (page) {
+		page = balloon_retrieve(highmem);
+		if (page && PageHighMem(page) == highmem) {
 			pages[pgno++] = page;
 		} else {
 			enum bp_state st;
-			st = decrease_reservation(nr_pages - pgno, GFP_HIGHUSER);
+			if (page)
+				balloon_append(page);
+			st = decrease_reservation(nr_pages - pgno,
+					highmem ? GFP_HIGHUSER : GFP_USER);
 			if (st != BP_DONE)
 				goto out_undo;
 		}

drivers/xen/gntdev.c

@@ -123,7 +123,7 @@ static struct grant_map *gntdev_alloc_map(struct gntdev_priv *priv, int count)
 	    NULL == add->pages)
 		goto err;
 
-	if (alloc_xenballooned_pages(count, add->pages))
+	if (alloc_xenballooned_pages(count, add->pages, false /* lowmem */))
 		goto err;
 
 	for (i = 0; i < count; i++) {

include/xen/balloon.h

@@ -25,8 +25,9 @@ extern struct balloon_stats balloon_stats;
 
 void balloon_set_new_target(unsigned long target);
 
-int alloc_xenballooned_pages(int nr_pages, struct page** pages);
-void free_xenballooned_pages(int nr_pages, struct page** pages);
+int alloc_xenballooned_pages(int nr_pages, struct page **pages,
+		bool highmem);
+void free_xenballooned_pages(int nr_pages, struct page **pages);
 
 struct sys_device;
 #ifdef CONFIG_XEN_SELFBALLOONING