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 <[email protected]>
Cc: Izik Eidus <[email protected]>
Cc: Andrea Arcangeli <[email protected]>
Cc: Nick Piggin <[email protected]>
Cc: KOSAKI Motohiro <[email protected]>
Reviewed-by: Rik van Riel <[email protected]>
Cc: Lee Schermerhorn <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: KAMEZAWA Hiroyuki <[email protected]>
Cc: Wu Fengguang <[email protected]>
Cc: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

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;