linux-kernel - Re: [PATCH] Tight check of pfn_valid on sparsemem

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Date:	Fri, 30 Jul 2010 07:48:00 -0500 (CDT)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
cc:	Russell King - ARM Linux <linux@....linux.org.uk>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Milton Miller <miltonm@....com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mel@....ul.ie>,
	Johannes Weiner <hannes@...xchg.org>,
	Kukjin Kim <kgene.kim@...sung.com>
Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4

On Thu, 29 Jul 2010, Dave Hansen wrote:

> SPARSEMEM_EXTREME would be a bit different.  It's a 2-level lookup.
> You'd have 16 "section roots", each representing 256MB of address space.
> Each time we put memory under one of those roots, we'd fill in a
> 512-section second-level table, which is designed to always fit into one
> page.  If you start at 256MB, you won't waste all those entries.

That is certain a solution to the !MMU case and it would work very much
like a page table. If you have an MMU then the vmemmap sparsemem
configuration can take advantage of of that to avoid the 2 level lookup.
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