Subject: Re: [PATCH] Tight check of pfn_valid on sparsemem - v4
From: Dave Hansen <dave@linux.vnet.ibm.com>
To: Christoph Lameter <cl@linux-foundation.org>
Cc: Russell King - ARM Linux <linux@arm.linux.org.uk>,
        Minchan Kim <minchan.kim@gmail.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>,
        Milton Miller <miltonm@bga.com>, linux-kernel@vger.kernel.org,
        linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>,
        Mel Gorman <mel@csn.ul.ie>, Johannes Weiner <hannes@cmpxchg.org>,
        Kukjin Kim <kgene.kim@samsung.com>
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Date: Fri, 30 Jul 2010 08:43:54 -0700
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On Fri, 2010-07-30 at 07:48 -0500, Christoph Lameter wrote:
> 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.

Yup, couldn't agree more, Christoph.

It wouldn't hurt to have several them available on ARM since the
architecture is so diverse.

-- Dave

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