If you’ve seen following error
>>> Emerging (1 of 1) .... mount: /proc: cannot mount none read-only. Unable to mark /proc slave: 32 * The ebuild phase 'die_hooks' has been aborted since PORTAGE_BUILDDIR * does not exist: '/var/tmp/portage/ ...'
while emerging some package, than be sure this is a bug in portage. Just append
to make.conf, and it should work again.