[PATCH 1/6] mm: define PAGE_MAPPING_FLAGS

Hugh Dickins
2009-11-10T21:51:56Z

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>
---

include/linux/ksm.h | 5 +++--
include/linux/mm.h | 17 ++++++++++++++++-
include/linux/rmap.h | 8 ++++++++
mm/migrate.c | 11 ++++-------
mm/rmap.c | 7 +++----
mm/swapfile.c | 2 +-
6 files changed, 35 insertions(+), 15 deletions(-)

--- mm0/include/linux/ksm.h 2009-09-28 00:28:38.000000000 +0100
+++ mm1/include/linux/ksm.h 2009-11-04 10:52:45.000000000 +0000
@@ -38,7 +38,8 @@ static inline void ksm_exit(struct mm_st
*/
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 *p
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);
}
}
--- mm0/include/linux/mm.h 2009-11-02 12:32:34.000000000 +0000
+++ mm1/include/linux/mm.h 2009-11-04 10:52:45.000000000 +0000
@@ -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
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;
--- mm0/include/linux/rmap.h 2009-09-28 00:28:39.000000000 +0100
+++ mm1/include/linux/rmap.h 2009-11-04 10:52:45.000000000 +0000
@@ -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;
--- mm0/mm/migrate.c 2009-11-02 12:32:34.000000000 +0000
+++ mm1/mm/migrate.c 2009-11-04 10:52:45.000000000 +0000
@@ -172,17 +172,14 @@ static void remove_anon_migration_ptes(s
{
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)
--- mm0/mm/rmap.c 2009-11-02 12:32:34.000000000 +0000
+++ mm1/mm/rmap.c 2009-11-04 10:52:45.000000000 +0000
@@ -203,7 +203,7 @@ struct anon_vma *page_lock_anon_vma(stru

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
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 ||
@@ -512,7 +511,7 @@ 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);
--- mm0/mm/swapfile.c 2009-11-04 10:21:17.000000000 +0000
+++ mm1/mm/swapfile.c 2009-11-04 10:52:45.000000000 +0000
@@ -937,7 +937,7 @@ static int unuse_vma(struct vm_area_stru
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;