Re: [PATCH] crash: Default to CRASH_DUMP=n when support for it is unlikely

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Dave,

On Fri, Aug 23, 2024 at 2:54 PM Dave Vasilevsky <dave@vasilevsky.ca> wrote:
> Fixes boot failures on 6.9 on PPC_BOOK3S_32 machines using
> Open Firmware. On these machines, the kernel refuses to boot
> from non-zero PHYSICAL_START, which occurs when CRASH_DUMP is on.
>
> Since most PPC_BOOK3S_32 machines boot via Open Firmware, it should
> default to off for them. Users booting via some other mechanism
> can still turn it on explicitly.
>
> Also defaults to CRASH_DUMP=n on sh.
>
> Signed-off-by: Dave Vasilevsky <dave@vasilevsky.ca>
> Reported-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
> Closes: https://lists.debian.org/debian-powerpc/2024/07/msg00001.html
> Fixes: 75bc255a7444 ("crash: clean up kdump related config items")

Thanks for your patch!

> --- a/kernel/Kconfig.kexec
> +++ b/kernel/Kconfig.kexec
> @@ -97,7 +97,7 @@ config KEXEC_JUMP
>
>  config CRASH_DUMP
>         bool "kernel crash dumps"
> -       default y
> +       default ARCH_DEFAULT_CRASH_DUMP
>         depends on ARCH_SUPPORTS_CRASH_DUMP
>         depends on KEXEC_CORE
>         select VMCORE_INFO

IMHO CRASH_DUMP should just default to n, like most kernel options, as
it enables non-trivial extra functionality: the kernel source tree has
more than 100 locations that check if CONFIG_CRASH_DUMP is enabled.

Letting it default to enabled also conflicts with the spirit of the
help text for the symbol:

          Generate crash dump after being started by kexec.
          This should be normally only set in special crash dump kernels
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          which are loaded in the main kernel with kexec-tools into
          a specially reserved region and then later executed after
          a crash by kdump/kexec. The crash dump kernel must be compiled
          to a memory address not used by the main kernel or BIOS using
          PHYSICAL_START, or it must be built as a relocatable image
          (CONFIG_RELOCATABLE=y).
          For more details see Documentation/admin-guide/kdump/kdump.rst

          For s390, this option also enables zfcpdump.
          See also <file:Documentation/arch/s390/zfcpdump.rst>

What is so special about CRASH_DUMP, that it should be enabled by
default?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds