Re: [patch 48/91] kernel/crash_core: add crashkernel=auto for vmcore creation

[David Hildenbrand <david@redhat.com>]

On 07.05.21 03:04, Andrew Morton wrote:
> From: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
> Subject: kernel/crash_core: add crashkernel=auto for vmcore creation
> 
> This adds crashkernel=auto feature to configure reserved memory for vmcore
> creation.  CONFIG_CRASH_AUTO_STR is defined to be set for different kernel
> distributions and different archs based on their needs.
> 
> Link: https://lkml.kernel.org/r/20210223174153.72802-1-saeed.mirzamohammadi@oracle.com
> Signed-off-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@oracle.com>
> Signed-off-by: John Donnelly <john.p.donnelly@oracle.com>
> Tested-by: John Donnelly <john.p.donnelly@oracle.com>
> ed-by: Dave Young <dyoung@redhat.com>
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Vivek Goyal <vgoyal@redhat.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: Randy Dunlap <rdunlap@infradead.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
> Cc: Mike Kravetz <mike.kravetz@oracle.com>
> Cc: "Guilherme G. Piccoli" <gpiccoli@canonical.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> Cc: YiFei Zhu <yifeifz2@illinois.edu>
> Cc: Josh Poimboeuf <jpoimboe@redhat.com>
> Cc: Mike Rapoport <rppt@kernel.org>
> Cc: Masahiro Yamada <masahiroy@kernel.org>
> Cc: Sami Tolvanen <samitolvanen@google.com>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Christian Brauner <christian.brauner@ubuntu.com>
> Cc: Stephen Boyd <sboyd@kernel.org>
> Cc: Andrey Konovalov <andreyknvl@google.com>
> Cc: Colin Ian King <colin.king@canonical.com>
> Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> 
>   Documentation/admin-guide/kdump/kdump.rst       |    3 +-
>   Documentation/admin-guide/kernel-parameters.txt |    6 ++++
>   arch/Kconfig                                    |   20 ++++++++++++++
>   kernel/crash_core.c                             |    7 ++++
>   4 files changed, 35 insertions(+), 1 deletion(-)
> 
> --- a/arch/Kconfig~kernel-crash_core-add-crashkernel=auto-for-vmcore-creation
> +++ a/arch/Kconfig
> @@ -14,6 +14,26 @@ menu "General architecture-dependent opt
>   config CRASH_CORE
>   	bool
>   
> +config CRASH_AUTO_STR
> +	string "Memory reserved for crash kernel"
> +	depends on CRASH_CORE
> +	default "1G-64G:128M,64G-1T:256M,1T-:512M"
> +	help
> +	  This configures the reserved memory dependent
> +	  on the value of System RAM. The syntax is:
> +	  crashkernel=<range1>:<size1>[,<range2>:<size2>,...][@offset]
> +	              range=start-[end]
> +
> +	  For example:
> +	      crashkernel=512M-2G:64M,2G-:128M
> +
> +	  This would mean:
> +
> +	      1) if the RAM is smaller than 512M, then don't reserve anything
> +	         (this is the "rescue" case)
> +	      2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
> +	      3) if the RAM size is larger than 2G, then reserve 128M
> +
>   config KEXEC_CORE
>   	select CRASH_CORE
>   	bool
> --- a/Documentation/admin-guide/kdump/kdump.rst~kernel-crash_core-add-crashkernel=auto-for-vmcore-creation
> +++ a/Documentation/admin-guide/kdump/kdump.rst
> @@ -285,7 +285,8 @@ This would mean:
>       2) if the RAM size is between 512M and 2G (exclusive), then reserve 64M
>       3) if the RAM size is larger than 2G, then reserve 128M
>   
> -
> +Or you can use crashkernel=auto to choose the crash kernel memory size
> +based on the recommended configuration set for each arch.
>   
>   Boot into System Kernel
>   =======================
> --- a/Documentation/admin-guide/kernel-parameters.txt~kernel-crash_core-add-crashkernel=auto-for-vmcore-creation
> +++ a/Documentation/admin-guide/kernel-parameters.txt
> @@ -751,6 +751,12 @@
>   			a memory unit (amount[KMG]). See also
>   			Documentation/admin-guide/kdump/kdump.rst for an example.
>   
> +	crashkernel=auto
> +			[KNL] This parameter will set the reserved memory for
> +			the crash kernel based on the value of the CRASH_AUTO_STR
> +			that is the best effort estimation for each arch. See also
> +			arch/Kconfig for further details.
> +
>   	crashkernel=size[KMG],high
>   			[KNL, X86-64] range could be above 4G. Allow kernel
>   			to allocate physical memory region from top, so could
> --- a/kernel/crash_core.c~kernel-crash_core-add-crashkernel=auto-for-vmcore-creation
> +++ a/kernel/crash_core.c
> @@ -7,6 +7,7 @@
>   #include <linux/crash_core.h>
>   #include <linux/utsname.h>
>   #include <linux/vmalloc.h>
> +#include <linux/kexec.h>
>   
>   #include <asm/page.h>
>   #include <asm/sections.h>
> @@ -250,6 +251,12 @@ static int __init __parse_crashkernel(ch
>   	if (suffix)
>   		return parse_crashkernel_suffix(ck_cmdline, crash_size,
>   				suffix);
> +#ifdef CONFIG_CRASH_AUTO_STR
> +	if (strncmp(ck_cmdline, "auto", 4) == 0) {
> +		ck_cmdline = CONFIG_CRASH_AUTO_STR;
> +		pr_info("Using crashkernel=auto, the size chosen is a best effort estimation.\n");
> +	}
> +#endif
I remember that the original "crashkernel=auto" as once proposed by Red 
Hat people did not receive a warm welcome.

Let me take a look .... oh, there it is from 2009

https://marc.info/?t=125006512600002&r=1&w=2

and then we had it in 2018

https://lkml.org/lkml/2018/5/20/262


The issue I have with this: it's just plain wrong when you take memory 
hotplug into serious account as we see it quite heavily in VMs. You 
don't know what you'll need when building a kernel. Just pass it via the 
cmdline ...

-- 
Thanks,

David / dhildenb