linux-kernel - Re: [PATCH 2/2][v2] powerpc: Make the CMM memory hotplug aware

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Message-ID: <alpine.DEB.1.10.0910161246010.6120@gentwo.org>
Date:	Fri, 16 Oct 2009 12:48:34 -0400 (EDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	gerald.schaefer@...ibm.com
cc:	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	linuxppc-dev@...abs.org, Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Mel Gorman <mel@....ul.ie>, Ingo Molnar <mingo@...e.hu>,
	Badari Pulavarty <pbadari@...ibm.com>,
	Brian King <brking@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Paul Mackerras <paulus@...ba.org>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 2/2][v2] powerpc: Make the CMM memory hotplug aware

On Thu, 15 Oct 2009, Gerald Schaefer wrote:

> > The pages allocated as __GFP_MOVABLE are used to store the list of pages
> > allocated by the balloon.  They reference virtual addresses and it would
> > be fine for the kernel to migrate the physical pages for those, the
> > balloon would not notice this.
>
> Does page migration really work for kernel pages that were allocated
> with __get_free_page()? I was wondering if we can do this on s390, where
> we have a 1:1 mapping of kernel virtual to physical addresses, but
> looking at migrate_pages() and friends, it seems that kernel pages
> w/o mapping and rmap should not be migrateable at all. Any thoughts from
> the memory migration experts?

page migration only works for pages where we have some way of accounting
for all the references to a page. This usually mean using reverse mappings
(anon list, radix trees and page tables).


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