make compound page destructor handling explicit

index cd0528d..667a72e 100644
@@ -298,6 +298,24 @@ void put_pages_list(struct list_head *pa
 void split_page(struct page *page, unsigned int order);
 
 /*
+ * Compound pages have a destructor function.  Provide a 
+ * prototype for that function and accessor functions.
+ * These are _only_ valid on the head of a PG_compound page.
+ */
+typedef void compound_page_dtor(struct page *);
+
+static inline void set_compound_page_dtor(struct page *page,
+						compound_page_dtor *dtor)
+{
+	page[1].lru.next = (void *)dtor;
+}
+
+static inline compound_page_dtor *get_compound_page_dtor(struct page *page)
+{
+	return (compound_page_dtor *)page[1].lru.next;
+}
+
+/*
  * Multiple processes may "see" the same page. E.g. for untouched
  * mappings of /dev/null, all processes see the same page full of
  * zeroes, and text pages of executables and shared libraries have

index 2911a36..2032fb2 100644
@@ -88,6 +88,8 @@ static struct page *dequeue_huge_page(st
 	return page;
 }
 
+/* declare the destructor to catch a missmatch on definition. */
+static compound_page_dtor free_huge_page;
 static void free_huge_page(struct page *page)
 {
 	BUG_ON(page_count(page));

@@ -109,7 +111,7 @@ static int alloc_fresh_huge_page(void)
 	if (nid == MAX_NUMNODES)
 		nid = first_node(node_online_map);
 	if (page) {
-		page[1].lru.next = (void *)free_huge_page;	/* dtor */
+		set_compound_page_dtor(page, free_huge_page);
 		spin_lock(&hugetlb_lock);
 		nr_huge_pages++;
 		nr_huge_pages_node[page_to_nid(page)]++;

index 7938e46..113d9bc 100644
@@ -240,6 +240,8 @@ static void bad_page(struct page *page)
  * This usage means that zero-order pages may not be compound.
  */
 
+/* declare the destructor to catch a missmatch on definition. */
+static compound_page_dtor free_compound_page;
 static void free_compound_page(struct page *page)
 {
 	__free_pages_ok(page, (unsigned long)page[1].lru.prev);

@@ -250,7 +252,7 @@ static void prep_compound_page(struct pa
 	int i;
 	int nr_pages = 1 << order;
 
-	page[1].lru.next = (void *)free_compound_page;	/* set dtor */
+	set_compound_page_dtor(page, free_compound_page);
 	page[1].lru.prev = (void *)order;
 	for (i = 0; i < nr_pages; i++) {
 		struct page *p = page + i;

index 6cc9cb0..87456b2 100644
@@ -58,9 +58,9 @@ static void put_compound_page(struct pag
 {
 	page = (struct page *)page_private(page);
 	if (put_page_testzero(page)) {
-		void (*dtor)(struct page *page);
+		compound_page_dtor *dtor;
 
-		dtor = (void (*)(struct page *))page[1].lru.next;
+		dtor = get_compound_page_dtor(page);
 		(*dtor)(page);
 	}
 }