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

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Message-ID: <1279038933.10995.9.camel@nimitz>
Date:	Tue, 13 Jul 2010 09:35:33 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Johannes Weiner <hannes@...xchg.org>, linux@....linux.org.uk,
	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,
	Mel Gorman <mel@....ul.ie>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [RFC] Tight check of pfn_valid on sparsemem

On Wed, 2010-07-14 at 00:43 +0900, Minchan Kim wrote:
> 3 is not a big deal than 2 about memory usage.
> If the system use memory space fully(MAX_PHYSMEM_BITS 31), it just consumes
> 1024(128 * 8) byte. So now I think best solution is 2. 
> 
> Russell. What do you think about it? 

I'm not Russell, but I'll tell you what I think. :)

Make the sections 16MB.  You suggestion to add the start/end pfns
_doubles_ the size of the structure, and its size overhead.  We have
systems with a pretty tremendous amount of memory with 16MB sections.

If you _really_ can't make the section size smaller, and the vast
majority of the sections are fully populated, you could hack something
in.  We could, for instance, have a global list that's mostly readonly
which tells you which sections need to be have their sizes closely
inspected.  That would work OK if, for instance, you only needed to
check a couple of memory sections in the system.  It'll start to suck if
you made the lists very long.

-- Dave

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