I have a test_and_set_bit(PG_chainlock, page->flags) in page reclaim.
Which works fine on SMP.  But on uniprocessor, we made
pte_chain_unlock() a no-op, so all pages end up with PG_chainlock set.
refill_inactive() cannot move any pages onto the inactive list and the
machine dies.

The patch removes the test_and_set_bit optimisation in there and just
uses pte_chain_lock().  If we want that (dubious) optimisation back
then let's do it right and create pte_chain_trylock().