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>

include/linux/page-flags.h

@@ -87,7 +87,6 @@ enum pageflags {
 	PG_private_2,		/* If pagecache, has fs aux data */
 	PG_writeback,		/* Page is under writeback */
 	PG_head,		/* A head page */
-	PG_swapcache,		/* Swap page: swp_entry_t in private */
 	PG_mappedtodisk,	/* Has blocks allocated on-disk */
 	PG_reclaim,		/* To be reclaimed asap */
 	PG_swapbacked,		/* Page is backed by RAM/swap */
@@ -110,6 +109,9 @@ enum pageflags {
 	/* Filesystems */
 	PG_checked = PG_owner_priv_1,
 
+	/* SwapBacked */
+	PG_swapcache = PG_owner_priv_1,	/* Swap page: swp_entry_t in private */
+
 	/* Two page bits are conscripted by FS-Cache to maintain local caching
 	 * state.  These bits are set on pages belonging to the netfs's inodes
 	 * when those inodes are being locally cached.
@@ -314,7 +316,13 @@ PAGEFLAG_FALSE(HighMem)
 #endif
 
 #ifdef CONFIG_SWAP
-PAGEFLAG(SwapCache, swapcache, PF_NO_COMPOUND)
+static __always_inline int PageSwapCache(struct page *page)
+{
+	return PageSwapBacked(page) && test_bit(PG_swapcache, &page->flags);
+
+}
+SETPAGEFLAG(SwapCache, swapcache, PF_NO_COMPOUND)
+CLEARPAGEFLAG(SwapCache, swapcache, PF_NO_COMPOUND)
 #else
 PAGEFLAG_FALSE(SwapCache)
 #endif
@@ -701,12 +709,12 @@ static inline void ClearPageSlabPfmemalloc(struct page *page)
  * Flags checked when a page is freed.  Pages being freed should not have
  * these flags set.  It they are, there is a problem.
  */
-#define PAGE_FLAGS_CHECK_AT_FREE \
-	(1UL << PG_lru	 | 1UL << PG_locked    | \
-	 1UL << PG_private | 1UL << PG_private_2 | \
-	 1UL << PG_writeback | 1UL << PG_reserved | \
-	 1UL << PG_slab	 | 1UL << PG_swapcache | 1UL << PG_active | \
-	 1UL << PG_unevictable | __PG_MLOCKED)
+#define PAGE_FLAGS_CHECK_AT_FREE				\
+	(1UL << PG_lru		| 1UL << PG_locked	|	\
+	 1UL << PG_private	| 1UL << PG_private_2	|	\
+	 1UL << PG_writeback	| 1UL << PG_reserved	|	\
+	 1UL << PG_slab		| 1UL << PG_active 	|	\
+	 1UL << PG_unevictable	| __PG_MLOCKED)
 
 /*
  * Flags checked when a page is prepped for return by the page allocator.

include/trace/events/mmflags.h

@@ -95,7 +95,6 @@
 	{1UL << PG_private_2,		"private_2"	},		\
 	{1UL << PG_writeback,		"writeback"	},		\
 	{1UL << PG_head,		"head"		},		\
-	{1UL << PG_swapcache,		"swapcache"	},		\
 	{1UL << PG_mappedtodisk,	"mappedtodisk"	},		\
 	{1UL << PG_reclaim,		"reclaim"	},		\
 	{1UL << PG_swapbacked,		"swapbacked"	},		\

mm/memory-failure.c

@@ -764,12 +764,11 @@ static int me_huge_page(struct page *p, unsigned long pfn)
  */
 
 #define dirty		(1UL << PG_dirty)
-#define sc		(1UL << PG_swapcache)
+#define sc		((1UL << PG_swapcache) | (1UL << PG_swapbacked))
 #define unevict		(1UL << PG_unevictable)
 #define mlock		(1UL << PG_mlocked)
 #define writeback	(1UL << PG_writeback)
 #define lru		(1UL << PG_lru)
-#define swapbacked	(1UL << PG_swapbacked)
 #define head		(1UL << PG_head)
 #define slab		(1UL << PG_slab)
 #define reserved	(1UL << PG_reserved)
@@ -819,7 +818,6 @@ static struct page_state {
 #undef mlock
 #undef writeback
 #undef lru
-#undef swapbacked
 #undef head
 #undef slab
 #undef reserved

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 */