[PATCH] cleanup and generalise lowmem_page_address

From: William Lee Irwin III <wli@holomorphy.com>

This patch allows architectures to micro-optimize lowmem_page_address() at
their whims.  Roman Zippel originally wrote and/or suggested this back when
dependencies on page->virtual existing were being shaken out.  That's
long-settled, so it's fine to do this now.

include/linux/mm.h

@@ -339,9 +339,14 @@ static inline void set_page_zone(struct page *page, unsigned long zone_num)
 	page->flags |= zone_num << ZONE_SHIFT;
 }
 
-static inline void * lowmem_page_address(struct page *page)
+#ifndef CONFIG_DISCONTIGMEM
+/* The array of struct pages - for discontigmem use pgdat->lmem_map */
+extern struct page *mem_map;
+#endif
+
+static inline void *lowmem_page_address(struct page *page)
 {
-	return __va( ( (page - page_zone(page)->zone_mem_map)	+ page_zone(page)->zone_start_pfn) << PAGE_SHIFT);
+	return __va(page_to_pfn(page) << PAGE_SHIFT);
 }
 
 #if defined(CONFIG_HIGHMEM) && !defined(WANT_PAGE_VIRTUAL)
@@ -395,11 +400,6 @@ static inline int page_mapped(struct page *page)
 #define VM_FAULT_MINOR	1
 #define VM_FAULT_MAJOR	2
 
-#ifndef CONFIG_DISCONTIGMEM
-/* The array of struct pages - for discontigmem use pgdat->lmem_map */
-extern struct page *mem_map;
-#endif 
-
 extern void show_free_areas(void);
 
 struct page *shmem_nopage(struct vm_area_struct * vma,