Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

[Eric DeVolder]

On 10/28/22 12:06, Borislav Petkov wrote:
> On Fri, Oct 28, 2022 at 10:29:45AM -0500, Eric DeVolder wrote:
> > So it is with this in mind that I suggest we stay with the statically sized elfcorehdr buffer.
> >
> > If that can be agreed upon, then it is "just a matter" of picking a useful
> > elfcorehdr size. Currently that size is derived from the NR_DEFAULT_CPUS and
> > CRASH_MAX_MEMORY_RANGES. So, there is still the CRASH_MAX_MEMORY_RANGES knob
> > to help a dial in size, should there be some issue with the default
> > value/size.
>
> Let's see
>
>          kbuf.memsz =
>                  (CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES) *
>                          sizeof(Elf64_Phdr);
>
> which, IINM, is
>
> 	(8192 + 32768) * 56
>
> which is something like 2M.
>
> (CONFIG_NR_CPUS_DEFAULT = 8192 - this is because of MAXSMP which gets
> set on distro kernels)
>
> Now, since userspace kexec tools uses 2048 for max memory ranges, that
> size becomes smaller - around half a Mb. And since y'all wanna be on the
> safe side, you can quadruple it and have
>
> 	(8192 + 8192) * 56
>
> which is still under a megabyte. And that's fine, I guess, on a big
> server.

Excellent, I'll set CRASH_MAX_MEMORY_RANGES to 8192! That seems a quite fair trade off of elfcorehdr 
size vs system size (ie 1TiB w/ 128MiB memblock size).

> > Or if there is desire to drop computing the size from NR_DEFAULT_CPUs and
>
> I think you should leave the dependency on the Kconfig size so that
> smaller machines which are configured this way, don't end up wasting
> unnecessary memory.

Excellent, I'll leave the computation as NR_DEFAULT_CPUS + CRASH_MAX_MEMORY_RANGES.

> > It is my intention to correct the CRASH_MAX_MEMORY_RANGES (if we keep it) as such:
> >
> > config CRASH_MAX_MEMORY_RANGES
> >      depends on CRASH_DUMP && KEXEC_FILE && MEMORY_HOTPLUG
>
> Yes, but don't leave it to the user to decide what number to choose
> - choose a high enough number, explain why you've chosen this with a
> comment and that's it.

I currently have the Kconfig item as:

config CRASH_MAX_MEMORY_RANGES
    depends on CRASH_DUMP && KEXEC_FILE && MEMORY_HOTPLUG
    int
    default 8192
    help
      For the kexec_file_load path, specify the maximum number of
      memory regions, eg. as represented by the 'System RAM' entries
      in /proc/iomem, that the elfcorehdr buffer/segment can accommodate.
      This value is combined with NR_CPUS and multiplied by Elf64_Phdr
      size to determine the final buffer size.

I'll work to provide information a better explanation as to the 8192 number.

Thank you!
eric

> Thx.