Troubleshoot Linux VM boot issues due to kernel panic

This document includes troubleshooting information about a VM becoming unresponsive due to kernel panic errors.

Before you begin

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Kernel panic

A kernel panic can happen when the kernel is unable to load properly initramfs modules, which are required for the guest OS to boot.

Another form of kernel panic can occur in a situation where the kernel doesn't know how to handle a certain request and protects itself by stopping. Kernel panic can happen on a Compute Engine VM running RedHat, SUSE, CentOS, or Ubuntu.

Common error messages

Below are some of the most common kernel panic events for reference:

Kernel panic - not syncing: hung_task: blocked tasks

Kernel Panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)

Kernel panic - not syncing: NMI: Not continuing

Kernel panic - not syncing: out of memory. panic_on_oom is selected

Kernel panic - not syncing: Fatal Machine check

Common causes

The kernel panic error can occur due to multiple reasons. Some of the common reasons are:

The entry related to the initramfs file that corresponds to the kernel doesn't exist in the grub.cfg file.
The initramfs file doesn't get generated in the /boot directory during kernel installation.
The initramfs file gets only partially generated or is corrupted.

Symptoms

When you experience kernel panic on a VM instance, a common symptom is that the kernel does not allow you to connect to the VM, even when using the serial console.

You should check the serial console logs to identify the kernel that was loaded by the guest OS, for example:

[    0.000000] Initializing cgroup subsys cpu
[    0.000000] Initializing cgroup subsys cpuacct
[    0.000000] Linux version 3.10.0-1160.95.1.el7.x86_64 (mockbuild@x86-vm-42.build.eng.bos.redhat.com) (gcc version 4.8.5 20150623 (Red Hat 4.8.5-44) (GCC) ) #1 SMP Thu Aug 10 10:46:21 EDT 2023

Also check the kernel panic error. This error is normally seen either at the kernel line when the VM starts or at the end of the serial console logs with multiple stack call traces.

The following example shows a kernel panic event due to initramfs issues:

[    1.520840] No filesystem could mount root, tried:
[    1.520840]
[    1.521964] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[    1.523495] CPU: 1 PID: 1 Comm: swapper/0 Not tainted 3.10.0-1160.95.1.el7.x86_64 #1
[    1.524932] Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/11/2022
[    1.526901] Call Trace:
[    1.527421]  dump_stack+0x41/0x60
[    1.527978]  panic+0xe7/0x2ac
[    1.528578]  mount_block_root+0x2be/0x2e6
[    1.529693]  ? do_early_param+0x95/0x95
[    1.530441]  prepare_namespace+0x135/0x16b
[    1.531237]  kernel_init_freeable+0x203/0x22d
[    1.532081]  ? rest_init+0xaa/0xaa
[    1.532808]  kernel_init+0xa/0x103
[    1.533395]  ret_from_fork+0x35/0x40
[    1.535229] Kernel Offset: 0x23a00000 from 0xffffffff81000000

Resolve the kernel panic error

To resolve the kernel panic error, perform the following steps:

Connect to the serial console and log in to the VM from the Google Cloud console.
Click Reset for VM in the Google Cloud console.
After the GRUB splash screen appears, select the previously working kernel or rescue kernel, and then boot the system. This causes the VM to start with the selected kernel.

Note: Booting the VM by selecting the previous kernel from the GRUB splash screen doesn't make changes to GRUB configuration file, so this is not a persistent change.
When the VM is accessible, you can initiate an SSH connection to the VM.
Identify the cause of the issue and take further action accordingly.

For example, if the initramfs file is missing or corrupted, complete the following steps:
1. Generate the initramfs file corresponding to original kernel by using the dracut command:
```
dracut -f /boot/initramfs-KERNEL_VERSION.img KERNEL_VERSION
```
  Replace KERNEL_VERSION with the VM's current kernel version. For example, 3.10.0-1160.95.1.el7.x86_64.
2. Update the grub2.cfg file using the grub2-mkconfig command, for example:
```
grub2-mkconfig -o /boot/grub2/grub.cfg
```
3. After the initramfs file is generated, you can restart the VM without any errors.