mm: fix KPF_SWAPCACHE in /proc/kpageflags

Commit 6326fec ("mm: Use owner_priv bit for PageSwapCache, valid
when PageSwapBacked") aliased PG_swapcache to PG_owner_priv_1 (and
depending on PageSwapBacked being true).

As a result, the KPF_SWAPCACHE bit in '/proc/kpageflags' should now be
synthesized, instead of being shown on unrelated pages which just happen
to have PG_owner_priv_1 set.

Signed-off-by: Hugh Dickins <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Wu Fengguang <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

Author: Hugh Dickins

fs/proc/page.c

@@ -173,7 +173,8 @@ u64 stable_page_flags(struct page *page)
 	u |= kpf_copy_bit(k, KPF_ACTIVE,	PG_active);
 	u |= kpf_copy_bit(k, KPF_RECLAIM,	PG_reclaim);
 
-	u |= kpf_copy_bit(k, KPF_SWAPCACHE,	PG_swapcache);
+	if (PageSwapCache(page))
+		u |= 1 << KPF_SWAPCACHE;
 	u |= kpf_copy_bit(k, KPF_SWAPBACKED,	PG_swapbacked);
 
 	u |= kpf_copy_bit(k, KPF_UNEVICTABLE,	PG_unevictable);