mm: check if any page in a pageblock is reserved before marking it MIGRATE_RESERVE

This fixes a problem where the first pageblock got marked MIGRATE_RESERVE
even though it only had a few free pages.  eg, On current ARM port, The
kernel starts at offset 0x8000 to leave room for boot parameters, and the
memory is freed later.

This in turn caused no contiguous memory to be reserved and frequent
kswapd wakeups that emptied the caches to get more contiguous memory.

Unfortunatelly, ARM needs order-2 allocation for pgd (see
arm/mm/pgd.c#pgd_alloc()).  Therefore the issue is not minor nor easy
avoidable.

[kosaki.motohiro@jp.fujitsu.com: added some explanation]
[kosaki.motohiro@jp.fujitsu.com: add !pfn_valid_within() to check]
[minchan.kim@gmail.com: check end_pfn in pageblock_is_reserved]
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Arve Hjønnevåg <arve@android.com>
Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Acked-by: Mel Gorman <mel@csn.ul.ie>
Acked-by: Dave Hansen <dave@linux.vnet.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/page_alloc.c

@@ -3328,6 +3328,20 @@ static inline unsigned long wait_table_bits(unsigned long size)
 
 #define LONG_ALIGN(x) (((x)+(sizeof(long))-1)&~((sizeof(long))-1))
 
+/*
+ * Check if a pageblock contains reserved pages
+ */
+static int pageblock_is_reserved(unsigned long start_pfn, unsigned long end_pfn)
+{
+	unsigned long pfn;
+
+	for (pfn = start_pfn; pfn < end_pfn; pfn++) {
+		if (!pfn_valid_within(pfn) || PageReserved(pfn_to_page(pfn)))
+			return 1;
+	}
+	return 0;
+}
+
 /*
  * Mark a number of pageblocks as MIGRATE_RESERVE. The number
  * of blocks reserved is based on min_wmark_pages(zone). The memory within
@@ -3337,7 +3351,7 @@ static inline unsigned long wait_table_bits(unsigned long size)
  */
 static void setup_zone_migrate_reserve(struct zone *zone)
 {
-	unsigned long start_pfn, pfn, end_pfn;
+	unsigned long start_pfn, pfn, end_pfn, block_end_pfn;
 	struct page *page;
 	unsigned long block_migratetype;
 	int reserve;
@@ -3367,7 +3381,8 @@ static void setup_zone_migrate_reserve(struct zone *zone)
 			continue;
 
 		/* Blocks with reserved pages will never free, skip them. */
-		if (PageReserved(page))
+		block_end_pfn = min(pfn + pageblock_nr_pages, end_pfn);
+		if (pageblock_is_reserved(pfn, block_end_pfn))
 			continue;
 
 		block_migratetype = get_pageblock_migratetype(page);