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.
> 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.
> 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.
Thx.
--
Regards/Gruss,
Boris.
https://people.kernel.org/tglx/notes-about-netiquette
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