7 Some pitfalls

If your new kernel does really weird things after a routine kernel upgrade, chances are you forgot to make clean before compiling the new kernel. Symptoms can be anything from your system outright crashing, strange I/O problems, to crummy performance. Make sure you do a make dep, too.

If your kernel is sucking up a lot of memory, is too large, or just takes forever to compile even when you've got your new 486DX6/440 working on it, you've probably got lots of unneeded stuff (device drivers, filesystems, etc) configured. If you don't use it, don't configure it, because it does take up memory. The most obvious symptom of kernel bloat is extreme swapping in and out of memory to disk; if your disk is making a lot of noise, look over your kernel configuration.

You can find out how much memory the kernel is using by taking the total amount of memory in your machine and subtracting it from the amount of ``total mem'' in /proc/meminfo or the output of the command `free'. You can also find out by doing a `dmesg' (or by looking at the kernel log file, wherever it is on your system). There will be a line which looks like this:

Memory: 15124k/16384k available (552k kernel code, 384k reserved, 324k data)

My 386 (which has slightly less junk configured) says this:

Memory: 7000k/8192k available (496k kernel code, 384k reserved, 312k data)

If it does not compile, then it is likely that a patch failed, or your source is somehow corrupt. Your version of gcc also might not be correct, or could also be corrupt (for example, the include files might be in error). Make sure that the symbolic links which Linus describes in the README are set up correctly. In general, if a standard kernel does not compile, something is seriously wrong with the system, and reinstallation of certain tools is probably necessary.

You did not run LILO, or it is not configured correctly. One thing that ``got'' me once was a problem in the config file; it said `boot = /dev/hda1' instead of `boot = /dev/hda' (This can be really annoying at first, but once you have a working config file, you shouldn't need to change it.).

Ooops! The best thing you can do here is to boot off of a floppy disk and prepare another bootable floppy (such as `make zdisk' would do). You need to know where your root (/) filesystem is and what type it is (e.g. second extended, minix). In the example below, you also need to know what filesystem your /usr/src/linux source tree is on, its type, and where it is normally mounted.

In the following example, / is /dev/hda1, and the filesystem which holds /usr/src/linux is /dev/hda3, normally mounted at /usr. Both are second extended filesystems. The working kernel image in /usr/src/linux/arch/i386/boot is called zImage.

The idea is that if there is a functioning zImage, it is possible to use that for the new floppy. Another alternative, which may or may not work better (it depends on the particular method in which you messed up your system) is discussed after the example.

First, boot from a boot/root disk combo or rescue disk, and mount the filesystem which contains the working kernel image:

mkdir /mnt
mount -t ext2 /dev/hda3 /mnt

If mkdir tells you that the directory already exists, just ignore it. Now, cd to the place where the working kernel image was. Note that

/mnt + /usr/src/linux/arch/i386/boot - /usr = /mnt/src/linux/arch/i386/boot

cd /mnt/src/linux/arch/i386/boot
dd if=zImage of=/dev/fd0
rdev /dev/fd0 /dev/hda1

cd to / and unmount the normal /usr filesystem:

cd /
umount /mnt

You should now be able to reboot your system as normal from this floppy. Don't forget to run lilo (or whatever it was that you did wrong) after the reboot!

As mentioned above, there is another common alternative. If you happened to have a working kernel image in / (/vmlinuz for example), you can use that for a boot disk. Supposing all of the above conditions, and that my kernel image is /vmlinuz, just make these alterations to the example above: change /dev/hda3 to /dev/hda1 (the / filesystem), /mnt/src/linux to /mnt, and if=zImage to if=vmlinuz. The note explaining how to derive /mnt/src/linux may be ignored.

This can be a severe problem. Starting with a kernel release after 1.0 (around 20 Apr 1994), a program called `update' which periodically flushes out the filesystem buffers, was upgraded/replaced. Get the sources to `bdflush' (you should find it where you got your kernel source), and install it (you probably want to run your system under the old kernel while doing this). It installs itself as `update' and after a reboot, the new kernel should no longer complain.

Get new versions of the route program and any other programs which do route manipulation. /usr/include/linux/route.h (which is actually a file in /usr/src/linux) has changed.

Upgrade to at least version 1.2.1.