It's easy to fix, here's the grub config before:
title Gentoo Linux
root (hd0,0)
kernel /boot/vmlinuz-3.1.6-gentoo
And fixed one:
title Gentoo Linux
root (hd0,0)
kernel /boot/vmlinuz-3.1.6-gentoo root=/dev/sda1
I had to pass an explicit root= parameter. How to figure it out? mount -l or cat /proc/mounts are not so helpful:
rootfs on / type rootfs (rw)
/dev/root on / type ext3 (rw,noatime,errors=continue,barrier=1,data=writeback)
So I used "fdisk -l" just to make sure whether it's sda or something else...
Device Boot Start End Blocks Id System
/dev/sda1 63 59006744 29503341 83 Linux
/dev/sda2 59006745 62910539 1951897+ 82 Linux swap / Solaris