ext4 Makefile - No targets. Stop

Well, the kernel headers are a different version from the kernel I built. The headers are the same as the original Ubuntu kernel version, 6.8.0-45, and the kernel I downloaded is 6.8.12.

I got the code with 'apt source linux-image-unsigned-$(uname -r)', but it didn't download the 6.8.0-45 code, which was my kernel at the time. I'm not familiar with how the different distros work, but I guess somewhere there is a discrepancy.

I finally found the 6.8.12 headers, although they are 6.8.12-common. I'll build the kernel again and hopefully the new headers will be used. It looks like they are installed

apt search linux-headers-$(uname -r) Sorting... Done Full Text Search... Done linux-headers-6.8.12-common/now 6.8.12-1 all [installed,local] Common header files for Linux 6.8.12

I built the kernel with the new kernel headers and I still can't insmod ext4.ko. I don't know... I give up.

Going forward I'll simply build the entire kernel after each code change and that's that. I'm not modifying anything else, so the kernel builds pretty quickly.

It's far from ideal, building the kernel every time I make a code change and then rebooting. Building ext4 alone and using it as a module would have been so much quicker. Maybe over time I'll find a better solution.

Ok, latest findings in case anybody ever searches for this issue. I still can't load the ext4 module alone, but at this point it's irrelevant. I found a way to not rebuild the kernel and reboot after each code change.

1. Make sure ext4 is not loaded with lsmod | grep ext4. If it is, run 'modprobe -r ext4'.
(If step 1 is skipped and the ext4 module is still loaded, the kernel will keep using the old version after steps 2 and 3. You'll wonder why you don't see the code changes in dmesg.)
2. Build ext4.ko in the source code directory ('make --debug=v -C /home/linux-6.8.0 M=/home/linux-6.8.0/fs/ext4')
3. Run 'make modules_install' (this will put the newly built ext4.ko in the /lib/modules/kernel version/kernel directory)

Now the new ext4 is available to modprobe and mount and the printk output displays in dmesg. Maybe I'll find out some day why ext4 can't be loaded as a standalone module, but that's just for my curiosity.