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[Xen-devel] Re: [PATCH 1/4] mm: Remove dependency on CONFIG_FLATMEM from online_page()

On Mon, 2011-05-02 at 23:19 +0200, Daniel Kiper wrote:
> Memory hotplug code strictly depends on CONFIG_SPARSEMEM.
> It means that code depending on CONFIG_FLATMEM in online_page()
> is never compiled. Remove it because it is not needed anymore.

It's subtle, but I don't think that's true.  We had another hotplug mode
for x86_64 before folks were comfortable turning SPARSEMEM on for the
whole architecture.  It was quite possible to have memory hotplug
without sparsemem in that case.  I think Keith Mannthey did some of that
code if I remember right.

But, I'm not sure how much of that stayed in distros versus made it
upstream.  In any case, you might want to chase down the
X86_64_ACPI_NUMA bit to make sure it can't be used with FLATMEM ever.

        bool "Allow for memory hot-add"
        depends on SPARSEMEM || X86_64_ACPI_NUMA
        depends on (IA64 || X86 || PPC_BOOK3S_64 || SUPERH || S390)

-- Dave

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