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Date: Mon, 12 Apr 2010 08:55:05 +0200
From: Ingo Molnar <mingo@elte.hu>
Subject: Re: [PATCH 00 of 41] Transparent Hugepage Support #17
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To: Avi Kivity <avi@redhat.com>
Cc: Nick Piggin <npiggin@suse.de>, Mike Galbraith <efault@gmx.de>, Jason Garrett-Glaser <darkshikari@gmail.com>, Andrea Arcangeli <aarcange@redhat.com>, Linus Torvalds <torvalds@linux-foundation.org>, Pekka Enberg <penberg@cs.helsinki.fi>, Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org, Marcelo Tosatti <mtosatti@redhat.com>, Adam Litke <agl@us.ibm.com>, Izik Eidus <ieidus@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Rik van Riel <riel@redhat.com>, Mel Gorman <mel@csn.ul.ie>, Dave Hansen <dave@linux.vnet.ibm.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Mike Travis <travis@sgi.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Christoph Lameter <cl@linux-foundation.org>, Chris Wright <chrisw@sous-sol.org>, bpicco@redhat.com, KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>, Balbir Singh <balbir@linux.vnet.ibm.com>, Arnd Bergmann <arnd@arndb.de>, "Michael S. Tsirkin" <mst@redhat.com>, Peter Zijlstra <peterz@infradead.org>, Johannes Weiner <hannes@cmpxchg.org>, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
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* Avi Kivity <avi@redhat.com> wrote:

> > Defrag is not futile in theory, you just have to either have a reserve of 
> > movable pages (and never allow pinned kernel pages in there), or you need 
> > to allocate pinned kernel memory in units of the chunk size goal (which 
> > just gives you different types of fragmentation problems) or you need to 
> > do non-linear kernel mappings so you can defrag pinned kernel memory (with 
> > *lots* of other problems of course). So you just have a lot of downsides.
> 
> Non-linear kernel mapping moves the small page problem from userspace back 
> to the kernel, a really unhappy solution.

Note that in a theoretical sense a specific variant of non-linear kernel 
mappings is already implemented here and toda and is productized: it's called 
virtualization.

	Ingo

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