[PATCH] mm: correct the comment about the value for buddy _mapcount

[Wang Sheng-Hui <shhuiw@gmail.com>]

Set _mapcount PAGE_BUDDY_MAPCOUNT_VALUE to make the page buddy.
Not the magic number -2.

Signed-off-by: Wang Sheng-Hui <shhuiw@gmail.com>
---
 mm/page_alloc.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index b100255..345f57b 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -488,8 +488,10 @@ __find_buddy_index(unsigned long page_idx, unsigned int order)
  * (c) a page and its buddy have the same order &&
  * (d) a page and its buddy are in the same zone.
  *
- * For recording whether a page is in the buddy system, we set ->_mapcount -2.
- * Setting, clearing, and testing _mapcount -2 is serialized by zone->lock.
+ * For recording whether a page is in the buddy system, we set ->_mapcount
+ * PAGE_BUDDY_MAPCOUNT_VALUE.
+ * Setting, clearing, and testing _mapcount PAGE_BUDDY_MAPCOUNT_VALUE is
+ * serialized by zone->lock.
  *
  * For recording page's order, we use page_private(page).
  */
@@ -527,8 +529,9 @@ static inline int page_is_buddy(struct page *page, struct page *buddy,
  * as necessary, plus some accounting needed to play nicely with other
  * parts of the VM system.
  * At each level, we keep a list of pages, which are heads of continuous
- * free pages of length of (1 << order) and marked with _mapcount -2. Page's
- * order is recorded in page_private(page) field.
+ * free pages of length of (1 << order) and marked with _mapcount
+ * PAGE_BUDDY_MAPCOUNT_VALUE. Page's order is recorded in page_private(page)
+ * field.
  * So when we are allocating or freeing one, we can derive the state of the
  * other.  That is, if we allocate a small block, and both were
  * free, the remainder of the region must be split into blocks.
-- 
1.7.10.4

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