[solved] Booting up FreeBSD from Linux GRUB2

1. reinstall the freebsd,
(1) changed the geometry (by answering 'No' to the question that freebsd installer asked),
(2) let the / partition be the 1st partition in the freebsd slice, in other words, in freebsd slice, first create the / (root partition), then create the swap space.
2. boot up, as usual, you'll have problem on the pretty grub screen, so press 'c' to enter the command line mode.
3.
(1) insmod ufs2
(2) set root=(hd0,3,a)
(3) chainloader +1
(4) boot
4. you're free