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, 8 insertions, 12 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 576c25ee..367272d 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 1a0ee6e..f06cee4 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 d5eb2e8..e74112e 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;