From: Khalid Aziz <redacted>
Date: Mon, 7 Mar 2016 11:24:54 -0700

Tags can be cleared by user by setting tag to 0. Tags are
automatically cleared by the hardware when the mapping for a virtual
address is removed from TSB (which is why swappable pages are a
problem), so kernel does not have to do it as part of clean up.

You might be able to crib some bits for the Tag in the swp_entry_t, it's
64-bit and you can therefore steal bits from the offset field.

That way you'll have the ADI tag in the page tables, ready to re-install
at swapin time.