[PATCH] mm/pageblock: throw compiling time error if pageblock_bits can not hold MIGRATE_TYPES

[Pingfan Liu <kernelfans@gmail.com>]

Currently, NR_PAGEBLOCK_BITS and MIGRATE_TYPES are not associated by code.
If someone adds extra migrate type, then he may forget to enlarge the
NR_PAGEBLOCK_BITS.
NR_PAGEBLOCK_BITS depends on MIGRATE_TYPES, while these macro
spread on two different .h file with reverse dependency, it is a little
hard to refer to MIGRATE_TYPES in pageblock-flag.h. This patch tries to
remind such relation in compiling-time.

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
---
 include/linux/pageblock-flags.h | 5 +++--
 mm/page_alloc.c                 | 2 +-
 2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/linux/pageblock-flags.h b/include/linux/pageblock-flags.h
index 9132c5c..fe0aec4 100644
--- a/include/linux/pageblock-flags.h
+++ b/include/linux/pageblock-flags.h
@@ -25,11 +25,12 @@
 
 #include <linux/types.h>
 
+#define PB_migratetype_bits 3
 /* Bit indices that affect a whole block of pages */
 enum pageblock_bits {
 	PB_migrate,
-	PB_migrate_end = PB_migrate + 3 - 1,
-			/* 3 bits required for migrate types */
+	PB_migrate_end = PB_migrate + PB_migratetype_bits - 1,
+			/* n bits required for migrate types */
 	PB_migrate_skip,/* If set the block is skipped by compaction */
 
 	/*
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 2ec9cc4..537020f 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -425,7 +425,7 @@ void set_pfnblock_flags_mask(struct page *page, unsigned long flags,
 	unsigned long bitidx, word_bitidx;
 	unsigned long old_word, word;
 
-	BUILD_BUG_ON(NR_PAGEBLOCK_BITS != 4);
+	BUILD_BUG_ON(order_base_2(MIGRATE_TYPES) != PB_migratetype_bits);
 
 	bitmap = get_pageblock_bitmap(page, pfn);
 	bitidx = pfn_to_bitidx(page, pfn);
-- 
2.7.4