[PATCH] stop using "base" argument in __free_pages_bulk()

Appended is a patch which stops using the zone->zone_mem_map to calculate
the buddy and combined page pointers.  It uses the fact that the mem_map
array is guaranteed to be contigious for the surrounding (1 << MAX_ORDER)
pages.  The relative positions of the pages in the physical address space
to provide the alignement; which conicidentally fixes the issue where zones
are not aligned at MAX_ORDER.  There is a very comprehensive comment in the
new code explaining the mathematical relationship between a page and its
buddy so I won't reproduce it here.

This kind of approach is required for CONFIG_NONLINEAR systems where the
mem_map is not contiguous within a zone, and the zone->zone_mem_map is not
used at all.

This patch has been boot-tested on a large variety of systems and
architectures: my P4 laptop, 16-way NUMAQ, 16-way Summit, 4-way x86 SMP,
ppc64 LPAR, x86_64, and several ia64 configurations.

It has been performance-tested on a 16-way NUMAQ.  SDET shows a very slight
(within margin of error) performance gain.  Kernbench shows an
approximately ~1% decrease in system time with this patch applied.  So, it
has a likely positive performance impact.

However, the patch has the potential to have a negative performance impact
on systems with an expensive page_to_pfn() implementation.  But, I think
the NUMAQ has one of the more expensive ones around, and it doesn't seem
mind too much.
Signed-off-by: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Dave Hansen <haveblue@us.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

mm/page_alloc.c

@@ -190,6 +190,37 @@ static inline void rmv_page_order(struct page *page)
 	page->private = 0;
 }
 
+/*
+ * Locate the struct page for both the matching buddy in our
+ * pair (buddy1) and the combined O(n+1) page they form (page).
+ *
+ * 1) Any buddy B1 will have an order O twin B2 which satisfies
+ * the following equation:
+ *     B2 = B1 ^ (1 << O)
+ * For example, if the starting buddy (buddy2) is #8 its order
+ * 1 buddy is #10:
+ *     B2 = 8 ^ (1 << 1) = 8 ^ 2 = 10
+ *
+ * 2) Any buddy B will have an order O+1 parent P which
+ * satisfies the following equation:
+ *     P = B & ~(1 << O)
+ *
+ * Assumption: *_mem_map is contigious at least up to MAX_ORDER
+ */
+static inline struct page *
+__page_find_buddy(struct page *page, unsigned long page_idx, unsigned int order)
+{
+	unsigned long buddy_idx = page_idx ^ (1 << order);
+
+	return page + (buddy_idx - page_idx);
+}
+
+static inline unsigned long
+__find_combined_index(unsigned long page_idx, unsigned int order)
+{
+	return (page_idx & ~(1 << order));
+}
+
 /*
  * This function checks whether a page is free && is the buddy
  * we can do coalesce a page and its buddy if
@@ -233,44 +264,43 @@ static inline int page_is_buddy(struct page *page, int order)
  * -- wli
  */
 
-static inline void __free_pages_bulk (struct page *page, struct page *base,
+static inline void __free_pages_bulk (struct page *page,
 		struct zone *zone, unsigned int order)
 {
 	unsigned long page_idx;
-	struct page *coalesced;
 	int order_size = 1 << order;
 
 	if (unlikely(order))
 		destroy_compound_page(page, order);
 
-	page_idx = page - base;
+	page_idx = page_to_pfn(page) & ((1 << MAX_ORDER) - 1);
 
 	BUG_ON(page_idx & (order_size - 1));
 	BUG_ON(bad_range(zone, page));
 
 	zone->free_pages += order_size;
 	while (order < MAX_ORDER-1) {
+		unsigned long combined_idx;
 		struct free_area *area;
 		struct page *buddy;
-		int buddy_idx;
 
-		buddy_idx = (page_idx ^ (1 << order));
-		buddy = base + buddy_idx;
+		combined_idx = __find_combined_index(page_idx, order);
+		buddy = __page_find_buddy(page, page_idx, order);
+
 		if (bad_range(zone, buddy))
 			break;
 		if (!page_is_buddy(buddy, order))
-			break;
-		/* Move the buddy up one level. */
+			break;		/* Move the buddy up one level. */
 		list_del(&buddy->lru);
 		area = zone->free_area + order;
 		area->nr_free--;
 		rmv_page_order(buddy);
-		page_idx &= buddy_idx;
+		page = page + (combined_idx - page_idx);
+		page_idx = combined_idx;
 		order++;
 	}
-	coalesced = base + page_idx;
-	set_page_order(coalesced, order);
-	list_add(&coalesced->lru, &zone->free_area[order].free_list);
+	set_page_order(page, order);
+	list_add(&page->lru, &zone->free_area[order].free_list);
 	zone->free_area[order].nr_free++;
 }
 
@@ -309,10 +339,9 @@ free_pages_bulk(struct zone *zone, int count,
 		struct list_head *list, unsigned int order)
 {
 	unsigned long flags;
-	struct page *base, *page = NULL;
+	struct page *page = NULL;
 	int ret = 0;
 
-	base = zone->zone_mem_map;
 	spin_lock_irqsave(&zone->lock, flags);
 	zone->all_unreclaimable = 0;
 	zone->pages_scanned = 0;
@@ -320,7 +349,7 @@ free_pages_bulk(struct zone *zone, int count,
 		page = list_entry(list->prev, struct page, lru);
 		/* have to delete it as __free_pages_bulk list manipulates */
 		list_del(&page->lru);
-		__free_pages_bulk(page, base, zone, order);
+		__free_pages_bulk(page, zone, order);
 		ret++;
 	}
 	spin_unlock_irqrestore(&zone->lock, flags);