[PATCH] Align the node_mem_map endpoints to a MAX_ORDER boundary

Andy added code to buddy allocator which does not require the zone's
endpoints to be aligned to MAX_ORDER.  An issue is that the buddy allocator
requires the node_mem_map's endpoints to be MAX_ORDER aligned.  Otherwise
__page_find_buddy could compute a buddy not in node_mem_map for partial
MAX_ORDER regions at zone's endpoints.  page_is_buddy will detect that
these pages at endpoints are not PG_buddy (they were zeroed out by bootmem
allocator and not part of zone).  Of course the negative here is we could
waste a little memory but the positive is eliminating all the old checks
for zone boundary conditions.

SPARSEMEM won't encounter this issue because of MAX_ORDER size constraint
when SPARSEMEM is configured.  ia64 VIRTUAL_MEM_MAP doesn't need the logic
either because the holes and endpoints are handled differently.  This
leaves checking alloc_remap and other arches which privately allocate for
node_mem_map.

Signed-off-by: Bob Picco <bob.picco@hp.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Cc: Dave Hansen <haveblue@us.ibm.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

2 files changed, 12 insertions, 3 deletions

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index b5c21122c299..36740354d4db 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -22,6 +22,7 @@
 #else
 #define MAX_ORDER CONFIG_FORCE_MAX_ZONEORDER
 #endif
+#define MAX_ORDER_NR_PAGES (1 << (MAX_ORDER - 1))
 
 struct free_area {
         struct list_head        free_list;
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index bb3416932ab0..253a450c400d 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -2125,14 +2125,22 @@ static void __init alloc_node_mem_map(struct pglist_data *pgdat)
 #ifdef CONFIG_FLAT_NODE_MEM_MAP
         /* ia64 gets its own node_mem_map, before this, without bootmem */
         if (!pgdat->node_mem_map) {
-                unsigned long size;
+                unsigned long size, start, end;
                 struct page *map;
 
-                size = (pgdat->node_spanned_pages + 1) * sizeof(struct page);
+                /*
+                 * The zone's endpoints aren't required to be MAX_ORDER
+                 * aligned but the node_mem_map endpoints must be in order
+                 * for the buddy allocator to function correctly.
+                 */
+                start = pgdat->node_start_pfn & ~(MAX_ORDER_NR_PAGES - 1);
+                end = pgdat->node_start_pfn + pgdat->node_spanned_pages;
+                end = ALIGN(end, MAX_ORDER_NR_PAGES);
+                size =  (end - start) * sizeof(struct page);
                 map = alloc_remap(pgdat->node_id, size);
                 if (!map)
                         map = alloc_bootmem_node(pgdat, size);
-                pgdat->node_mem_map = map;
+                pgdat->node_mem_map = map + (pgdat->node_start_pfn - start);
         }
 #ifdef CONFIG_FLATMEM
         /*