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

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Date:   Fri, 28 Oct 2022 19:06:01 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Eric DeVolder <eric.devolder@...cle.com>
Cc:     Baoquan He <bhe@...hat.com>, david@...hat.com,
        Oscar Salvador <osalvador@...e.de>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        kexec@...ts.infradead.org, ebiederm@...ssion.com,
        dyoung@...hat.com, vgoyal@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, dave.hansen@...ux.intel.com, hpa@...or.com,
        nramas@...ux.microsoft.com, thomas.lendacky@....com,
        robh@...nel.org, efault@....de, rppt@...nel.org,
        sourabhjain@...ux.ibm.com, linux-mm@...ck.org
Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support

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