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Date: Thu, 8 Apr 2010 19:58:47 +0200
From: Andrea Arcangeli <aarcange@redhat.com>
Subject: Re: [PATCH 56 of 67] Memory compaction core
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To: Johannes Weiner <hannes@cmpxchg.org>
Cc: linux-mm@kvack.org, Andrew Morton <akpm@linux-foundation.org>, Marcelo Tosatti <mtosatti@redhat.com>, Adam Litke <agl@us.ibm.com>, Avi Kivity <avi@redhat.com>, Izik Eidus <ieidus@redhat.com>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Nick Piggin <npiggin@suse.de>, 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>, Ingo Molnar <mingo@elte.hu>, 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>, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>, Chris Mason <chris.mason@oracle.com>
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On Thu, Apr 08, 2010 at 07:56:04PM +0200, Johannes Weiner wrote:
> Humm, maybe the start pfn could be huge page aligned?  That would make
> it possible to check for PageTransHuge() and skip over compound_order()
> pages.  This way, we should never actually run into PG_tail pages.

The problem here are random compound pages that aren't owned by the
transparent hugepage subsystem. If we can't identify those, it's
unsafe to call compound_order (like it's unsafe to call page_order for
pagebuddy pages).

I don't see a way to solve this without a new PG_ bitflag. We could do
a 64-bit only optimization though... ;)

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