@jameshibbard brought up an important point: if you use either full-drive encryption, or encrypt your initramfs output (squashfs vmlinuz pacakges in /boot) as is a new feature, please be careful as switching out versions of your initramfs-tools seem likely to make your system unbootable.
Either way, as long as the driver unpins the pages upon mmu notifier callback,then there is proper synchronization with both filesystem and mm(page_mkclean(), munmap(), etc). Therefore, neither flag needs to be set.