How can I generate /proc/vmcore?

Question

I want to practice with makedumpfile. However, it needs /proc/vmcore which is the memory image of the currently running kernel.

Also, reading the man page of makedumpfile, we also need 2 kernels: panicked kernel (crashed kernel) and capture kernel. Does this capture kernel run on the same machine or remotely?

Answer 1

The capture kernel runs on the same host. It runs in memory that the panic'd kernel reserved for the capture kernel to use. The capture kernel is started with the kexec mechanism by the panicing kernel.

/proc/vmcore should be provided by the kernel if its setup to export a memory image. If your kernel does not have a /proc/vmcore, then you're missing the right kernel infrastructure.

The linux kernel source implies that /proc/vmcore is only populated inside a capture kernel (the kernel command line providing the address of the panic'd kernels vmcore ELF header is required), so /proc/vmcore will exist in a regular kernel, but won't contain anything at all.

Inside the capture kernel, /proc/vmcore presents the crashed kernel as an ELF core image.

Here's some RH doc with more details: https://access.redhat.com/knowledge/solutions/6038