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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: Wang Sheng-Hui <shhuiw@gmail.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Wang Sheng-Hui 2013-09-11 14:22:48 -07:00 committed by Linus Torvalds
parent fa0f281cf9
commit cf6fe94538
1 changed files with 7 additions and 4 deletions
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mm/page_alloc.c
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@ -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.