diff -rubB kernel-source-3.1.orig/mm/page_alloc.c kernel-source-3.1/mm/page_alloc.c
--- kernel-source-3.1.orig/mm/page_alloc.c	2011-10-24 10:10:05.000000000 +0300
+++ kernel-source-3.1/mm/page_alloc.c	2011-12-01 21:46:38.000000000 +0300
@@ -3371,8 +3371,16 @@
 	int reserve;
 
 	/* Get the start pfn, end pfn and the number of blocks to reserve */
+	/*
+	* Get the start pfn, end pfn and the number of blocks to reserve
+	* We have to be careful to be aligned to pageblock_nr_pages to
+	* make sure that we always check pfn_valid for the first page in
+	* the block.
+	*/
+
 	start_pfn = zone->zone_start_pfn;
 	end_pfn = start_pfn + zone->spanned_pages;
+	start_pfn = roundup(start_pfn, pageblock_nr_pages);
 	reserve = roundup(min_wmark_pages(zone), pageblock_nr_pages) >>
 							pageblock_order;