linux-kernel - Re: [PATCH 1/4] mm: Remove dependency on CONFIG_FLATMEM from online

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Date:	Tue, 3 May 2011 00:18:57 +0200
From:	Daniel Kiper <dkiper@...-space.pl>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
Cc:	Daniel Kiper <dkiper@...-space.pl>, ian.campbell@...rix.com,
	akpm@...ux-foundation.org, andi.kleen@...el.com,
	haicheng.li@...ux.intel.com, fengguang.wu@...el.com,
	jeremy@...p.org, konrad.wilk@...cle.com,
	dan.magenheimer@...cle.com, v.tolstov@...fip.ru, pasik@....fi,
	wdauchy@...il.com, rientjes@...gle.com,
	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: [PATCH 1/4] mm: Remove dependency on CONFIG_FLATMEM from online_page()

On Mon, May 02, 2011 at 02:25:04PM -0700, Dave Hansen wrote:
> 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.
>
> config MEMORY_HOTPLUG
>         bool "Allow for memory hot-add"
>         depends on SPARSEMEM || X86_64_ACPI_NUMA
>         depends on HOTPLUG && ARCH_ENABLE_MEMORY_HOTPLUG
>         depends on (IA64 || X86 || PPC_BOOK3S_64 || SUPERH || S390)

IIRC some time ago it was possible to enable memory hotplug with
CONFIG_FLATMEM. That is why I looked for any dependencies of memory
hoplug code on CONFIG_FLATMEM in current Linux Kernel source. I could
not find anything and that is why I published this patch. However,
maybe I missed something.

Daniel
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