Kdump is a standrad Linux mechanism to dump machine memory content on kernel crash. Kdump is based on Kexec. Kdump utilizes two kernels: system kernel and dump capture kernel. System kernel is a normal kernel that is booted with special kdump-specific flags. We need to tell the system kernel to reserve some amount of physical memory where dump-capture kernel will be loaded. We need to load the dump capture kernel in advance because at the moment crash happens there is no way to read any data from disk because kernel is broken.
Once kernel crash happens the kernel crash handler uses Kexec mechanism to boot dump capture kernel. Please note that memory with system kernel is untouched and accessible from dump capture kernel as seen at the moment of crash. Once dump capture kernel is booted user can use /dev/vmcore file to get access to memory of crashed system kernel. The dump can be saved to disk or copied over network to some other machine for further investigation.
In real production environments system and dump capture kernel will be different - system kernel needs a lot of features and compiled with a many kernel flags/drivers. While dump capture kernel goal is to be minimalistic and take as small amount of memory as possible, e.g. dump capture kernel can be compiled without network support if we store memory dump to disk only. But in this article we will simplify things and use the same kernel both as system and dump capture one. In means we will load the same kernel code twice - one as normal system kernel, another one to reserved memory area.
System/dump capture kernel requires some configuration flags that are not set by default. Please consult Kernels#Compilation for more information about compiling custom kernel in Arch. Here we will emphasize on Kdump specific configuration.
To create a kernel you need to edit kernel config (or config.x86_64) file and enable following configuration options:
CONFIG_DEBUG_INFO=y CONFIG_CRASH_DUMP=y CONFIG_PROC_VMCORE=y
Also change package base name to something like linux-kdump to distinguish the kernel from the default Arch one. Compile kernel package and install it.
In case if you have separate kernel for system and dump capture then it is recommended to consult Kdump documentation. It has several recommendations how to make dump capture kernel smaller.
Setup kdump kernel
First you need to reserve memory for dump capture kernel. Edit you bootloader config file and add crashkernel=64M kernel boot option. 64M of memory should be enough to hadle crash dumps on machines with up to 12G of RAM.
Next you need to tell Kexec that you want to use your dump capture kernel.
kexec -p /[path-to-capture-kernel] --append="root=[root-device] single irqpoll maxcpus=1 reset_devices"
It loads the kernel into reserved area. Without -p flag kexec would boot the kernel right away, but in presence of the flag kernel will be loaded into reserved memory but boot postponed until crash.
Instead of runnig kexec manually you might want to setup Systemd service that will run kexec on boot:
[Unit] Description=Load dump capture kernel After=local-fs.target [Service] ExecStart=/sbin/kexec -p /[path-to-capture-kernel] --append="root=[root-device] single irqpoll maxcpus=1 reset_devices" Type=oneshot [Install] WantedBy=multi-user.target
Then enable the service
systemctl enable kdump
If you want to test crash then you can use sysrq for this. WARNING! kernel crash may destroy data on your disk, run it at your own risk!
echo c > /proc/sysrq-trigger
Once crash happens kexec will load your dump capture kernel.
Dump crashed kernel
Once booted into dump capture kernel you can read
/dev/vmcore file. It is recommended to dump core to a file and analyze it later.
dd if=/dev/vmcore of=~/crash.dump
or optionally you can copy the crash to other machine. Once dump is saved you can reboot machine into normal system kernel.
Analyzing core dump
You can use either gdb tool or special gdb extension called crash that can be found in AUR. Run crash as
crash <KERNEL> ~/crash.dump. Follow man crash or http://people.redhat.com/~anderson/crash_whitepaper/ for more information about debugging issues.