OK, thank you every one, it is my English comprehension problem, I misunderstanded " /boot does NOT have to be a filesystem ", I am so sorry for wasting everybody's time.
If I create some hard disk partitions: /boot, /, /usr, /tmp ... , of cause the kernel is in the /boot partition, /etc/fstab is in the / partition not in /boot partition, when the system boot, how...