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Date:   Fri, 30 Sep 2022 18:50:55 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Eric DeVolder <eric.devolder@oracle.com>,
        Oscar Salvador <osalvador@suse.de>,
        Andrew Morton <akpm@linux-foundation.org>, david@redhat.com
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        kexec@lists.infradead.org, ebiederm@xmission.com,
        dyoung@redhat.com, bhe@redhat.com, vgoyal@redhat.com,
        tglx@linutronix.de, mingo@redhat.com, dave.hansen@linux.intel.com,
        hpa@zytor.com, nramas@linux.microsoft.com, thomas.lendacky@amd.com,
        robh@kernel.org, efault@gmx.de, rppt@kernel.org,
        sourabhjain@linux.ibm.com, linux-mm@kvack.org
Subject: Re: [PATCH v12 7/7] x86/crash: Add x86 crash hotplug support
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On Fri, Sep 30, 2022 at 10:36:49AM -0500, Eric DeVolder wrote:
> > Your help text talks about System RAM entries in /proc/iomem which means
> > that those entries are present somewhere in the kernel and you can read
> > them out and do the proper calculations dynamically instead of doing the
> > static CONFIG_NR_CPUS_DEFAULT + CONFIG_CRASH_MAX_MEMORY_RANGES thing.
> 
> The intent is to compute the max size buffer needed to contain a maximum
> populated elfcorehdr, which is primarily based on the number of CPUs and
> memory regions. Thus far I (and others involved) have not found a kernel
> method to determine the maximum number of memory regions possible (if you
> are aware of one, please let me know!). Thus CONFIG_CRASH_MAX_MEMORY_RANGES
> was born (rather borrowed from kexec-tools).

Let's ask some mm folks.

mm folks, is there a way to enumerate all the memory regions a machine
has?

It looks to me like register_memory_resource() in mm/memory_hotplug.c
does register the resource so there should be a way to count that list
of resources or at least maintain a count somewhere so that kexec/crash
code can know how big its elfcodehdr buffer should be instead of doing a
clumsy Kconfig item where people would need to guess...

Hmm.

> > +#ifdef CONFIG_CRASH_MAX_MEMORY_RANGES
> So I think the use of CONFIG_CRASH_MAX_MEMORY_RANGES is not correct; it
> still needs to be based on the cpu or memory hotplug options.

You're kidding, right?

+config CRASH_MAX_MEMORY_RANGES
+	depends on CRASH_DUMP && KEXEC_FILE && (HOTPLUG_CPU || MEMORY_HOTPLUG)
	^^^^^^^^^^				^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

> > @@ -622,6 +622,15 @@ static int __init crash_save_vmcoreinfo_init(void)
> >   subsys_initcall(crash_save_vmcoreinfo_init);
> >   #if defined(CONFIG_HOTPLUG_CPU) || defined(CONFIG_MEMORY_HOTPLUG)
> > +
> > +void __weak *arch_map_crash_pages(unsigned long paddr, unsigned long size)
> > +{
> > +	return NULL;
> > +}
> > +
> > +void __weak arch_unmap_crash_pages(void **ptr) { }
> > +void __weak arch_crash_handle_hotplug_event(struct kimage *image, unsigned int hp_action) { }
> > +
> I was asked by Baoquan He to eliminate the use of __weak

Because?

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette