[RFC PATCH 2/4] mm: use mapcount for identifying zbud pages

[Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>]

Currently zbud pages do not have any flags set so it is not possible to
identify them during migration or compaction.

Implement PageZbud() by comparing page->_mapcount to -127 to distinguish
pages allocated by zbud. Just like PageBuddy() is implemented.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@xxxxxxxxxxx>
---
 include/linux/mm.h |   23 +++++++++++++++++++++++
 mm/zbud.c          |    4 ++++
 2 files changed, 27 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index f022460..b9ae6f2 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -440,6 +440,7 @@ static inline void init_page_count(struct page *page)
  * efficiently by most CPU architectures.
  */
 #define PAGE_BUDDY_MAPCOUNT_VALUE (-128)
+#define PAGE_ZBUD_MAPCOUNT_VALUE (-127)
 
 static inline int PageBuddy(struct page *page)
 {
@@ -458,6 +459,28 @@ static inline void __ClearPageBuddy(struct page *page)
 	atomic_set(&page->_mapcount, -1);
 }
 
+#ifdef CONFIG_ZBUD
+static inline int PageZbud(struct page *page)
+{
+	return atomic_read(&page->_mapcount) == PAGE_ZBUD_MAPCOUNT_VALUE;
+}
+
+static inline void SetPageZbud(struct page *page)
+{
+	VM_BUG_ON(atomic_read(&page->_mapcount) != -1);
+	atomic_set(&page->_mapcount, PAGE_ZBUD_MAPCOUNT_VALUE);
+}
+
+static inline void ClearPageZbud(struct page *page)
+{
+	VM_BUG_ON(!PageZbud(page));
+	atomic_set(&page->_mapcount, -1);
+}
+#else
+PAGEFLAG_FALSE(Zbud)
+#endif
+
+
 void put_page(struct page *page);
 void put_pages_list(struct list_head *pages);
 
diff --git a/mm/zbud.c b/mm/zbud.c
index aa9a15c..9267cd9 100644
--- a/mm/zbud.c
+++ b/mm/zbud.c
@@ -197,7 +197,10 @@ static void get_zbud_page(struct zbud_header *zhdr)
 static int put_zbud_page(struct zbud_header *zhdr)
 {
 	struct page *page = virt_to_page(zhdr);
+	VM_BUG_ON(!PageZbud(page));
+
 	if (put_page_testzero(page)) {
+		ClearPageZbud(page);
 		free_hot_cold_page(page, 0);
 		return 1;
 	}
@@ -307,6 +310,7 @@ int zbud_alloc(struct zbud_pool *pool, int size, gfp_t gfp,
 	 * don't increase the page count.
 	 */
 	zhdr = init_zbud_page(page);
+	SetPageZbud(page);
 	bud = FIRST;
 
 found:
-- 
1.7.9.5

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