Make it clear that GFP_ZERO does not work with atomic highmem allocations.

We use a non-atomic kmap() to clear the page, which cannot be
used from atomic contexts.

mm/page_alloc.c

@@ -591,19 +591,20 @@ void fastcall free_cold_page(struct page *page)
 	free_hot_cold_page(page, 1);
 }
 
-/*
- * Really, prep_compound_page() should be called from __rmqueue_bulk().  But
- * we cheat by calling it from here, in the order > 0 path.  Saves a branch
- * or two.
- */
-static inline void prep_zero_page(struct page *page, int order)
+static inline void prep_zero_page(struct page *page, int order, int gfp_flags)
 {
 	int i;
 
+	BUG_ON((gfp_flags & (__GFP_WAIT | __GFP_HIGHMEM)) == __GFP_HIGHMEM);
 	for(i = 0; i < (1 << order); i++)
 		clear_highpage(page + i);
 }
 
+/*
+ * Really, prep_compound_page() should be called from __rmqueue_bulk().  But
+ * we cheat by calling it from here, in the order > 0 path.  Saves a branch
+ * or two.
+ */
 static struct page *
 buffered_rmqueue(struct zone *zone, int order, int gfp_flags)
 {
@@ -640,7 +641,7 @@ buffered_rmqueue(struct zone *zone, int order, int gfp_flags)
 		prep_new_page(page, order);
 
 		if (gfp_flags & __GFP_ZERO)
-			prep_zero_page(page, order);
+			prep_zero_page(page, order, gfp_flags);
 
 		if (order && (gfp_flags & __GFP_COMP))
 			prep_compound_page(page, order);