mm: Use owner_priv bit for PageSwapCache, valid when PageSwapBacked

A page is not added to the swap cache without being swap backed,
so PageSwapBacked mappings can use PG_owner_priv_1 for PageSwapCache.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Acked-by: Hugh Dickins <hughd@google.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Bob Peterson <rpeterso@redhat.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: Andrew Lutomirski <luto@kernel.org>
Cc: Andreas Gruenbacher <agruenba@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 8 insertions, 6 deletions

diff --git a/mm/migrate.c b/mm/migrate.c
index 0ed24b1fa77b..87f4d0f81819 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -466,13 +466,15 @@ int migrate_page_move_mapping(struct address_space *mapping,
          */
         newpage->index = page->index;
         newpage->mapping = page->mapping;
-        if (PageSwapBacked(page))
-                __SetPageSwapBacked(newpage);
-
         get_page(newpage);      /* add cache reference */
-        if (PageSwapCache(page)) {
-                SetPageSwapCache(newpage);
-                set_page_private(newpage, page_private(page));
+        if (PageSwapBacked(page)) {
+                __SetPageSwapBacked(newpage);
+                if (PageSwapCache(page)) {
+                        SetPageSwapCache(newpage);
+                        set_page_private(newpage, page_private(page));
+                }
+        } else {
+                VM_BUG_ON_PAGE(PageSwapCache(page), page);
         }
 
         /* Move dirty while page refs frozen and newpage not yet exposed */