add pfn_valid_within helper for sub-MAX_ORDER hole detection

Generally we work under the assumption that memory the mem_map array is
contigious and valid out to MAX_ORDER_NR_PAGES block of pages, ie.  that if we
have validated any page within this MAX_ORDER_NR_PAGES block we need not check
any other.  This is not true when CONFIG_HOLES_IN_ZONE is set and we must
check each and every reference we make from a pfn.

Add a pfn_valid_within() helper which should be used when scanning pages
within a MAX_ORDER_NR_PAGES block when we have already checked the validility
of the block normally with pfn_valid().  This can then be optimised away when
we do not have holes within a MAX_ORDER_NR_PAGES block of pages.

Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Bob Picco <bob.picco@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/mmzone.h

@@ -784,6 +784,18 @@ void sparse_init(void);
 void memory_present(int nid, unsigned long start, unsigned long end);
 unsigned long __init node_memmap_size_bytes(int, unsigned long, unsigned long);
 
+/*
+ * If it is possible to have holes within a MAX_ORDER_NR_PAGES, then we
+ * need to check pfn validility within that MAX_ORDER_NR_PAGES block.
+ * pfn_valid_within() should be used in this case; we optimise this away
+ * when we have no holes within a MAX_ORDER_NR_PAGES block.
+ */
+#ifdef CONFIG_HOLES_IN_ZONE
+#define pfn_valid_within(pfn) pfn_valid(pfn)
+#else
+#define pfn_valid_within(pfn) (1)
+#endif
+
 #endif /* !__ASSEMBLY__ */
 #endif /* __KERNEL__ */
 #endif /* _LINUX_MMZONE_H */

mm/page_alloc.c

@@ -156,10 +156,8 @@ static int page_outside_zone_boundaries(struct zone *zone, struct page *page)
 
 static int page_is_consistent(struct zone *zone, struct page *page)
 {
-#ifdef CONFIG_HOLES_IN_ZONE
-	if (!pfn_valid(page_to_pfn(page)))
+	if (!pfn_valid_within(page_to_pfn(page)))
 		return 0;
-#endif
 	if (zone != page_zone(page))
 		return 0;
 
@@ -346,10 +344,8 @@ __find_combined_index(unsigned long page_idx, unsigned int order)
 static inline int page_is_buddy(struct page *page, struct page *buddy,
 								int order)
 {
-#ifdef CONFIG_HOLES_IN_ZONE
-	if (!pfn_valid(page_to_pfn(buddy)))
+	if (!pfn_valid_within(page_to_pfn(buddy)))
 		return 0;
-#endif
 
 	if (page_zone_id(page) != page_zone_id(buddy))
 		return 0;