In theory, there is nothing which guarantees that nobody will
mess with the page between "UnlockPage" and "deactivate_page"
(that is pretty hard to happen, I suppose, but anyway)

--- mm/vmscan.c.orig       Sun Jan 14 13:23:55 2001
+++ mm/vmscan.c    Sun Jan 14 13:24:16 2001

@@ -72,10 +72,10 @@
                swap_duplicate(entry);
                set_pte(page_table, swp_entry_to_pte(entry));
 drop_pte:
-               UnlockPage(page);
                mm->rss--;
                if (!page->age)
                        deactivate_page(page);
+               UnlockPage(page);
                page_cache_release(page);
                return;
        }