mm: define PAGE_MAPPING_FLAGS

At present we define PageAnon(page) by the low PAGE_MAPPING_ANON bit set
in page->mapping, with the higher bits a pointer to the anon_vma; and have
defined PageKsm(page) as that with NULL anon_vma.

But KSM swapping will need to store a pointer there: so in preparation for
that, now define PAGE_MAPPING_FLAGS as the low two bits, including
PAGE_MAPPING_KSM (always set along with PAGE_MAPPING_ANON, until some
other use for the bit emerges).

Declare page_rmapping(page) to return the pointer part of page->mapping,
and page_anon_vma(page) to return the anon_vma pointer when that's what it
is.  Use these in a few appropriate places: notably, unuse_vma() has been
testing page->mapping, but is better to be testing page_anon_vma() (cases
may be added in which flag bits are set without any pointer).

Signed-off-by: Hugh Dickins <hugh.dickins@tiscali.co.uk>
Cc: Izik Eidus <ieidus@redhat.com>
Cc: Andrea Arcangeli <aarcange@redhat.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Cc: Lee Schermerhorn <Lee.Schermerhorn@hp.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Wu Fengguang <fengguang.wu@intel.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

3 files changed, 27 insertions, 3 deletions

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index a485c14..1401a31 100644
--- a/include/linux/ksm.h
+++ b/include/linux/ksm.h
@@ -38,7 +38,8 @@ static inline void ksm_exit(struct mm_struct *mm)
  */
 static inline int PageKsm(struct page *page)
 {
-	return ((unsigned long)page->mapping == PAGE_MAPPING_ANON);
+	return ((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) ==
+				(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
 }
 
 /*
@@ -47,7 +48,7 @@ static inline int PageKsm(struct page *page)
 static inline void page_add_ksm_rmap(struct page *page)
 {
 	if (atomic_inc_and_test(&page->_mapcount)) {
-		page->mapping = (void *) PAGE_MAPPING_ANON;
+		page->mapping = (void *) (PAGE_MAPPING_ANON | PAGE_MAPPING_KSM);
 		__inc_zone_page_state(page, NR_ANON_PAGES);
 	}
 }
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 24c3956..1202cd3 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -620,13 +620,22 @@ void page_address_init(void);
 /*
  * On an anonymous page mapped into a user virtual memory area,
  * page->mapping points to its anon_vma, not to a struct address_space;
- * with the PAGE_MAPPING_ANON bit set to distinguish it.
+ * with the PAGE_MAPPING_ANON bit set to distinguish it.  See rmap.h.
+ *
+ * On an anonymous page in a VM_MERGEABLE area, if CONFIG_KSM is enabled,
+ * the PAGE_MAPPING_KSM bit may be set along with the PAGE_MAPPING_ANON bit;
+ * and then page->mapping points, not to an anon_vma, but to a private
+ * structure which KSM associates with that merged page.  See ksm.h.
+ *
+ * PAGE_MAPPING_KSM without PAGE_MAPPING_ANON is currently never used.
  *
  * Please note that, confusingly, "page_mapping" refers to the inode
  * address_space which maps the page from disk; whereas "page_mapped"
  * refers to user virtual address space into which the page is mapped.
  */
 #define PAGE_MAPPING_ANON	1
+#define PAGE_MAPPING_KSM	2
+#define PAGE_MAPPING_FLAGS	(PAGE_MAPPING_ANON | PAGE_MAPPING_KSM)
 
 extern struct address_space swapper_space;
 static inline struct address_space *page_mapping(struct page *page)
@@ -644,6 +653,12 @@ static inline struct address_space *page_mapping(struct page *page)
 	return mapping;
 }
 
+/* Neutral page->mapping pointer to address_space or anon_vma or other */
+static inline void *page_rmapping(struct page *page)
+{
+	return (void *)((unsigned long)page->mapping & ~PAGE_MAPPING_FLAGS);
+}
+
 static inline int PageAnon(struct page *page)
 {
 	return ((unsigned long)page->mapping & PAGE_MAPPING_ANON) != 0;
diff --git a/include/linux/rmap.h b/include/linux/rmap.h
index cb0ba70..1f65af4 100644
--- a/include/linux/rmap.h
+++ b/include/linux/rmap.h
@@ -39,6 +39,14 @@ struct anon_vma {
 
 #ifdef CONFIG_MMU
 
+static inline struct anon_vma *page_anon_vma(struct page *page)
+{
+	if (((unsigned long)page->mapping & PAGE_MAPPING_FLAGS) !=
+					    PAGE_MAPPING_ANON)
+		return NULL;
+	return page_rmapping(page);
+}
+
 static inline void anon_vma_lock(struct vm_area_struct *vma)
 {
 	struct anon_vma *anon_vma = vma->anon_vma;