How to compile and install kernel.org kernel on a separate partition and then setup dual boot

Question

That question: Compiling and installing a kernel.org kernel to a custom volume on disk partially answers my question, however, not completely!

I have a disk with 30GB capacity. It has 2 partitions

• 2GB partition (/dev/sda1) which contains Debian
• 28GB partition (/dev/sda2) which is completely empty (formatted, filesystem is ext4)

I want to compile and install kernel.org kernel (https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.78.tar.xz) on /dev/sda2 and setup dual boot between my Debian and new kernel

So far:

I mounted my second partition to /mnt (i.e. mount /dev/sda2 /mnt) and created a directory /src within it (i.e. I have /mnt/src directory)

When in /mnt/src,

I downloaded the kernel and extracted it

wget --no-check-certificate https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.10.78.tar.xz
unxz -v linux-5.10.78.tar.xz
tar xvf linux-5.10.78.tar

I also installed the necessary tools to be able to compile and install the kernel

apt-get update
apt-get install build-essential libncurses-dev bison flex libssl-dev libelf-dev

When in /mnt/src/linux-5.10.78

I copied my existing Debian kernel configuration to the current directory (i.e. cp -v /boot/config-$(uname -r) .config) and then ran make olddefconfig (with the idea to use my existing configuration and provide default values for everything else). I also unchecked the Avoid speculative indirect branches in kernel option because I hit You are building kernel with non-retpoline compiler, please update your compiler.. Stop. during make.

I then ran make -j 4 again and waited for it to finish.

After that I ran INSTALL_MOD_PATH=/mnt INSTALL_PATH=/mnt/boot make modules_install to install the necessary modules, following the suggestion from the question I have linked above.

My problem is that now when I run update-initramfs it fails as it can't find a /lib/modules/5.10.78 folder. That would obviously be the case as it is under /mnt/lib/modules/5.10.78. I tried providing different root path (using the -b option) but that didn't work. I also tried chroot, however, I don't really have the necessary setup/ executables in /mnt to chroot into it.

After "fixing" above, I expect running update-grub would discover the kernel image, the root filesystem for booting it and the configuration files and setup everything so that after reboot I am provided with both booting options (Debian and the new kernel).

Any help with both update-initramfs and the actual grub configuration afterwards would be greatly appreciated! I have found links here and there but they are always very high level overview of the process.

Answer 1

One of the easiest way to install a minimal Debian system is debootstrap bullseye /mnt where bullseye is the release you seek, and /mnt the path where you want it to be installed.

It needs a kernel, and your procedure seems correct. Initramfs or initrd is only required for complex boot sequence : typically a root directory in a RAID array. I have none on my server and it works fine. (Note that your root filesystem type mustn’t be compiled as a module but embedded in the kernel).

You should also add an entry in your /boot/grub/grub.cfg. Typically :

menuentry "Alternate Debian , kernel 5.10.78" {
        insmod part_msdos
        insmod ext2
        set root='hd0,msdos2'
        linux   /boot/vmlinuz-5.10.78 root=/dev/sda2 ro  
}

insmod part_msdos is needed if you have an old partition scheme. On UEFI system, it should be insmod part_gpt, but I don’t know them much. insmod ext2 permits grub to read an ext2 (or ext4) file system. set root= indicates on which partition grub should search /boot/vmlinuz-5.10.78 (check the actual name : it may vary), linux indicate it should boot the kernel which follows, root=/dev/sda2 indicates your future root directory.

If you really want a more tiny alternative system, perhaps Gentoo is a better way, but I know it only by name. You should start with debootstrap.