How to deal with a kernel panic

Introduction

Sometimes a kernel installation goes wrong and you have to backtrack.

There can be many reasons for this: insufficient space on the /boot partition, an interrupted installation, or some kind of problem with a third-party application.

Fortunately for us, there is always something we can do to save the day.

Try to reboot with the previous kernel

First thing to try is to reboot with the previous kernel.

• Restart the system.
• Once you have reached the GRUB 2 boot screen, move the selection to the menu entry corresponding to the previous kernel and press the enter key.

Once the system has restarted, it can be repaired.

If the system does not boot, try the rescue mode (see above).

Uninstall the broken kernel

The easiest way to do this is to uninstall the kernel version that is not working and then reinstall it.

Note

You cannot remove a kernel that you are running.

To show the version of currently running kernel:

uname -r

You can check the list of installed kernels:

dnf list installed kernel\* | sort -V

but this command is perhaps more practical, since it only returns packages that have several versions installed:

dnf repoquery --installed --installonly

To remove a specific kernel, you can use dnf, specifying the kernel version you retrieved earlier:

dnf remove kernel-core-<version>

Example:

dnf remove kernel-5.14.0-427.20.1.el9_4.x86_64

or use the dnf repoquery command:

dnf remove $(dnf repoquery --installed --installonly --latest=1)

You can now upgrade your system and try reinstalling the latest kernel version.

dnf update

Reboot and see if the new kernel works this time.

Rescue mode

Rescue mode corresponds to the old single-user mode.

Note

To enter in rescue mode, you have to provide the root password.

To enter rescue mode, the easiest way is to select the line starting with 0-rescue-* in the grub menu.

Another way is to edit any line of the grub menu (pressing 'e' key) and add systemd.unit=rescue.target at the end of the line that starts with linux and then press ctrl+x to boot the system into rescue mode.

Note

You are then in qwerty mode.

Once you are in rescue mode and you have enter the root password, you can then repair your system.

For that, you might need to configure a temporary IP address thanks to ip ad add ... (see network chapter of our admin guide).

Last chance: Anaconda Rescue Mode

If none of the above methods work, it is still possible to boot from the installation ISO and repair the system.

This method is not covered by this documentation.

System maintenance

Cleaning old kernel versions

You can remove old installed kernel packages, keeping only latest version and version of running kernel:

dnf remove --oldinstallonly

Limiting the number of installed kernels versions

We can limit the number of kernel versions by editing the /etc/yum.conf file and setting the installonly_limit variable:

installonly_limit=3

Note

You should always keep at least the latest kernel version and a backup version.

Author: Antoine Le Morvan

Contributors: Steven Spencer