18. UNEVICTABLE
19. HWPOISON
20. NOPAGE
+ 21. KSM
Short descriptions to the page flags:
20. NOPAGE
no page frame exists at the requested address
+21. KSM
+ identical memory pages dynamically shared between one or more processes
+
[IO related page flags]
1. ERROR IO error occurred
3. UPTODATE page has up-to-date data