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Commit cf6fe945 authored by Wang Sheng-Hui's avatar Wang Sheng-Hui Committed by Linus Torvalds
Browse files

mm: correct the comment about the value for buddy _mapcount 



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

Signed-off-by: default avatarWang Sheng-Hui <shhuiw@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent fa0f281c

mm/page_alloc.c

+7 −4
Original line number Diff line number Diff line
@@ -489,8 +489,10 @@ __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).

 */
@@ -528,8 +530,9 @@ 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.