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>

6 files changed, 35 insertions, 15 deletions

diff --git a/include/linux/ksm.h b/include/linux/ksm.h
index a485c14ecd5d..1401a313fa77 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 24c395694f4d..1202cd3121e1 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 cb0ba7032609..1f65af44c6d2 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;
diff --git a/mm/migrate.c b/mm/migrate.c
index 576c25eeb1ca..367272d04423 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -172,17 +172,14 @@ static void remove_anon_migration_ptes(struct page *old, struct page *new)
 {
         struct anon_vma *anon_vma;
         struct vm_area_struct *vma;
-        unsigned long mapping;
-
-        mapping = (unsigned long)new->mapping;
-
-        if (!mapping || (mapping & PAGE_MAPPING_ANON) == 0)
-                return;
 
         /*
          * We hold the mmap_sem lock. So no need to call page_lock_anon_vma.
          */
-        anon_vma = (struct anon_vma *) (mapping - PAGE_MAPPING_ANON);
+        anon_vma = page_anon_vma(new);
+        if (!anon_vma)
+                return;
+
         spin_lock(&anon_vma->lock);
 
         list_for_each_entry(vma, &anon_vma->head, anon_vma_node)
diff --git a/mm/rmap.c b/mm/rmap.c
index 1a0ee6e634c2..f06cee48eca7 100644
--- a/mm/rmap.c
+++ b/mm/rmap.c
@@ -203,7 +203,7 @@ struct anon_vma *page_lock_anon_vma(struct page *page)
 
         rcu_read_lock();
         anon_mapping = (unsigned long) page->mapping;
-        if (!(anon_mapping & PAGE_MAPPING_ANON))
+        if ((anon_mapping & PAGE_MAPPING_FLAGS) != PAGE_MAPPING_ANON)
                 goto out;
         if (!page_mapped(page))
                 goto out;
@@ -248,8 +248,7 @@ vma_address(struct page *page, struct vm_area_struct *vma)
 unsigned long page_address_in_vma(struct page *page, struct vm_area_struct *vma)
 {
         if (PageAnon(page)) {
-                if ((void *)vma->anon_vma !=
-                    (void *)page->mapping - PAGE_MAPPING_ANON)
+                if (vma->anon_vma != page_anon_vma(page))
                         return -EFAULT;
         } else if (page->mapping && !(vma->vm_flags & VM_NONLINEAR)) {
                 if (!vma->vm_file ||
@@ -513,7 +512,7 @@ int page_referenced(struct page *page,
                 referenced++;
 
         *vm_flags = 0;
-        if (page_mapped(page) && page->mapping) {
+        if (page_mapped(page) && page_rmapping(page)) {
                 if (PageAnon(page))
                         referenced += page_referenced_anon(page, mem_cont,
                                                                 vm_flags);
diff --git a/mm/swapfile.c b/mm/swapfile.c
index d5eb2e85600b..e74112e8e5f4 100644
--- a/mm/swapfile.c
+++ b/mm/swapfile.c
@@ -938,7 +938,7 @@ static int unuse_vma(struct vm_area_struct *vma,
         unsigned long addr, end, next;
         int ret;
 
-        if (page->mapping) {
+        if (page_anon_vma(page)) {
                 addr = page_address_in_vma(page, vma);
                 if (addr == -EFAULT)
                         return 0;