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27 January 2022

What's Wrong with crashkernel=auto

by Petr Tesařík

The Problem

The “crash kernel” is a complete self-contained OS that is booted upon a kernel crash. Usually, its sole function is to save an image of the crashed kernel somewhere it can be analyzed later. Since it saves the content of all memory used by the crashed kernel, it cannot use that same memory for its own operation. That’s why it needs its own separate RAM region, not used by anything else.

So, how much RAM should be reserved for this crash kernel? Too much and your precious RAM is effectively wasted. Too little and dumping will fail on OOM. Besides, this is a large RAM region that must be continuous in physical memory, so it must be reserved early at boot, before memory gets too fragmented for such huge allocations. In other words, its size must be known before you even load the kernel. No wonder system admins are often frustrated by choosing a suitable number.

The Solution

Since the allocation happens at early boot time, the amount cannot be determined by a user-space application, because it’s already too late when the first user-space process starts. Red Hat engineers believe that they can solve it in the kernel, adding a crashkernel=auto option. It is implemented like this:

	if (strncmp(ck_cmdline, "auto", 4) == 0) {
#if defined(CONFIG_X86_64) || defined(CONFIG_S390)
		ck_cmdline = "1G-4G:160M,4G-64G:192M,64G-1T:256M,1T-:512M";
#elif defined(CONFIG_ARM64)
		ck_cmdline = "2G-:448M";
#elif defined(CONFIG_PPC64)
		char *fadump_cmdline;

		fadump_cmdline = get_last_crashkernel(cmdline, "fadump=", NULL);
		fadump_cmdline = fadump_cmdline ?
				fadump_cmdline + strlen("fadump=") : NULL;
		if (!fadump_cmdline || (strncmp(fadump_cmdline, "off", 3) == 0))
			ck_cmdline = "2G-4G:384M,4G-16G:512M,16G-64G:1G,64G-128G:2G,128G-:4G";
			ck_cmdline = "4G-16G:768M,16G-64G:1G,64G-128G:2G,128G-1T:4G,1T-2T:6G,2T-4T:12G,4T-8T:20G,8T-16T:36G,16T-32T:64G,32T-64T:128G,64T-:180G";
		pr_info("Using crashkernel=auto, the size chosen is a best effort estimation.\n");

Ouch! This code hardwires some arbitrary numbers into the kernel source… But even if it didn’t, can the kernel do any better?

How much memory is really needed? In other words, what must fit into RAM while saving the dump? Essentially, a complete single-purpose operating system. It usually consists of the currently installed kernel and selected user-space binaries. These days, the initrd image is usually created by dracut.

When booted after a crash, system RAM will contain:

None of the above is known when the original kernel reserves a crash kernel area. The panic kernel and initrd is loaded by kexec much later.

How should the original kernel know which files go into a crash initrd, and how big those files will be? What user-space processes will be running? Will that be systemd and a bunch of services, or merely a custom shell script? Will that script save the dump to an ext3 filesystem on an SD card, or to a remote NFSv4 share after configuring its own IP address with DHCP? More RAM is definitely needed in the latter case, right?

I have never found an easy way to estimate the size of kernel run-time data, not even from within the kernel itself. It is known to depend on a lot of things, memory size being only one of them. AFAIK nobody can even make a complete list of those things.

Oh, and you may even use a different crash kernel than the currently booting one.

I’m afraid, whatever guess can be made by the kernel at boot time, will be good only for one specific hardware and software configuration. But then it shouldn’t be in the kernel code.

tags: kdump