linux-kernel - Re: [PATCH v3] mm: make expand_downwards symmetrical to expand

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Date:	Wed, 20 Apr 2011 14:34:41 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Matthew Wilcox <matthew@....cx>
cc:	Pekka Enberg <penberg@...nel.org>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Michal Hocko <mhocko@...e.cz>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, linux-mm@...ck.org,
	LKML <linux-kernel@...r.kernel.org>,
	linux-parisc@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	x86 maintainers <x86@...nel.org>
Subject: Re: [PATCH v3] mm: make expand_downwards symmetrical to
 expand_upwards

On Wed, 20 Apr 2011, Matthew Wilcox wrote:

> > That part makes me think the best option is to make parisc do
> > CONFIG_NUMA as well regardless of the historical intent was.
> 
> But it's not just parisc.  It's six other architectures as well, some
> of which aren't even SMP.  Does !SMP && NUMA make any kind of sense?
> 

It does as long as DISCONTIGMEM is hijacking NUMA abstractions throughout 
the code; for example, look at the .config that James is probably using 
for testing here:

	CONFIG_PA8X00=y
	CONFIG_64BIT=y
	CONFIG_DISCONTIGMEM=y
	CONFIG_NEED_MULTIPLE_NODES=y
	CONFIG_NODES_SHIFT=3

and CONFIG_NUMA is not enabled.  So we want CONFIG_NODES_SHIFT of 3 
(because MAX_PHYSMEM_RANGES is 8) and CONFIG_NEED_MULTIPLE_NODES is 
enabled because of DISCONTIGMEM:

	#
	# Both the NUMA code and DISCONTIGMEM use arrays of pg_data_t's
	# to represent different areas of memory.  This variable allows
	# those dependencies to exist individually.
	#
	config NEED_MULTIPLE_NODES
		def_bool y
		depends on DISCONTIGMEM || NUMA

when in reality we should do away with CONFIG_NEED_MULTIPLE_NODES and just 
force DISCONTIGMEM to enable CONFIG_NUMA at least for -stable and as a 
quick fix for James.

In the long run, we'll probably want to define a lighterweight CONFIG_NUMA 
as a layer that CONFIG_DISCONTIGMEM can use for memory range abstractions 
and then CONFIG_NUMA is built on top of it to define proximity between 
those ranges.
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