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

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Message-ID: <20100729211310.GA28330@n2100.arm.linux.org.uk>
Date:	Thu, 29 Jul 2010 22:13:10 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Christoph Lameter <cl@...ux-foundation.org>
Cc:	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, Jul 29, 2010 at 02:55:53PM -0500, Christoph Lameter wrote:
> On Thu, 29 Jul 2010, Russell King - ARM Linux wrote:
> 
> > And no, setting the sparse section size to 512kB doesn't work - memory is
> > offset by 256MB already, so you need a sparsemem section array of 1024
> > entries just to cover that - with the full 256MB populated, that's 512
> > unused entries followed by 512 used entries.  That too is going to waste
> > memory like nobodies business.
> 
> SPARSEMEM EXTREME does not handle that?
> 
> Some ARMs seem to have MMUs. If so then use SPARSEMEM_VMEMMAP. You can map
> 4k pages for the mmap through a page table. Redirect unused 4k blocks to
> the NULL page.

We're going over old ground which has already been covered in this very
thread.  I've no compunction to repeat the arguments.
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