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Subject: Re: [PATCH 36 of 41] remove PG_buddy
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
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Date: Tue, 30 Mar 2010 07:18:47 +1100
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrea Arcangeli <aarcange@redhat.com>, 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>, 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>, Johannes Weiner <hannes@cmpxchg.org>, Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>
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On Mon, 2010-03-29 at 20:49 +0200, Peter Zijlstra wrote:
> On Mon, 2010-03-29 at 20:37 +0200, Andrea Arcangeli wrote:
> > From: Andrea Arcangeli <aarcange@redhat.com>
> > 
> > PG_buddy can be converted to page->_count == -1. So the PG_compound_lock can be
> > added to page->flags without overflowing (because of the section bits
> > increasing) with CONFIG_X86_PAE=y.
> 
> This seems to break the assumption that all free pages have a zero page
> count relied upon by things like page_cache_get_speculative().
> 
> What if a page-cache pages gets freed and used as a head in the buddy
> list while a concurrent lockless page-cache lookup tries to get a page
> ref?

And here goes me wanting to hijack it for PG_arch_2 :-)

Is there any other page flag we could hijack ? I need it only when the
page is allocated, so a flag that's only used when the page sits in the
buddy would be fine.

Cheers,
Ben.


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