After we compiled and installed the new Linux kernel, we found that we could no longer mount the /boot from a shell command. The computer booted fine.

In /etc/fstab, we have

/dev/sda1               /boot           ext2    noauto,noatime         1 2

In /boot/grub/menu.lst, we have

title=Gentoo Linux (2.6.28-gentoo-r5)
root (hd0,0)
kernel /kernel-2.6.28-gentoo-r5 root=/dev/sda3

Apparently, GRUB has no problem recognizing the ext2 filesystem, while kernel does not support ext2.

Check the file /usr/src/linux/.config, which we built using make menuconfig, we saw there was a line

CONFIG_EXT2_FS is not set

That explains it. Go back to make menuconfig, and find and check:

File systems --->
      <*> Second extended fs support
       [*]   Ext2 extended attributes
       [*]         Ext2 POSIX Access control
       [*]         Ext2Security Labels

Save the new config, run make && make modules_install again. After compilation finishes, you can boot into an older kernel that recognize ext2, and then you can mount /boot, and install the new kernel.

If you don't have an older kernel that recognize the ext2, you can use the installation CD to boot up, and then install the new kernel.

Reboot after you have installed the new kernel and test it out. This should have solved the problem.