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

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Date:	Sat, 31 Jul 2010 11:38:49 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Minchan Kim <minchan.kim@...il.com>
Cc:	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	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 Fri, Jul 30, 2010 at 06:32:04PM +0900, Minchan Kim wrote:
> On Fri, Jul 30, 2010 at 5:55 AM, Dave Hansen <dave@...ux.vnet.ibm.com> wrote:
> > If you free up parts of the mem_map[] array, how does the buddy
> > allocator still work?  I thought we required at 'struct page's to be
> > contiguous and present for at least 2^MAX_ORDER-1 pages in one go.

(Dave, I don't seem to have your mail to reply to.)

What you say is correct, and memory banks as a rule of thumb tend to be
powers of two.

We do have the ability to change MAX_ORDER (which we need to do for some
platforms where there's only 1MB of DMA-able memory.)

However, in the case of two 512KB banks, the buddy allocator won't try
to satisfy a 1MB request as it'll only have two separate 2x512K free
'pages' to deal with, and 0x1M free 'pages'.
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