linux-kernel - Re: [RFC] Tight check of pfn

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Message-ID: <20100713094612.GF20590@n2100.arm.linux.org.uk>
Date:	Tue, 13 Jul 2010 10:46:12 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Minchan Kim <minchan.kim@...il.com>,
	Yinghai Lu <yinghai@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Shaohua Li <shaohua.li@...el.com>,
	Yakui Zhao <yakui.zhao@...el.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	arm-kernel@...ts.infradead.org, kgene.kim@...sung.com,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC] Tight check of pfn_valid on sparsemem

On Tue, Jul 13, 2010 at 10:37:00AM +0100, Mel Gorman wrote:
> I prefer Kamezawa's suggestion of mapping on a ZERO_PAGE-like page full
> of PageReserved struct pages because it would have better performance
> and be more in line with maintaining the assumptions of the memory
> model. If we go in this direction, I would strongly prefer it was an
> ARM-only thing.

As I've said, this is not possible without doing some serious page
manipulation.

Plus the pages that where there become unusable as they don't correspond
with a PFN or obey phys_to_virt().  So there's absolutely no point to
this.

Now, why do we free the holes in the mem_map - because these holes can
be extremely large.  Every 512K of hole equates to one page of mem_map
array.  Balance that against memory placed at 0xc0000000 physical on
some platforms, and with PHYSMEM_BITS at 32 and SECTION_SIZE_BITS at
19 - well, you do the maths.  The result is certainly not pretty.
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