linux-kernel - Re: [PATCH 45 of 66] remove PG

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Message-ID: <20101209181553.GG19131@random.random>
Date:	Thu, 9 Dec 2010 19:15:53 +0100
From:	Andrea Arcangeli <aarcange@...hat.com>
To:	Mel Gorman <mel@....ul.ie>
Cc:	linux-mm@...ck.org, Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Adam Litke <agl@...ibm.com>, Avi Kivity <avi@...hat.com>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Rik van Riel <riel@...hat.com>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Ingo Molnar <mingo@...e.hu>, Mike Travis <travis@....com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Chris Wright <chrisw@...s-sol.org>, bpicco@...hat.com,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	"Michael S. Tsirkin" <mst@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
	Chris Mason <chris.mason@...cle.com>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 45 of 66] remove PG_buddy

On Thu, Nov 18, 2010 at 04:08:01PM +0000, Mel Gorman wrote:
> On Wed, Nov 03, 2010 at 04:28:20PM +0100, Andrea Arcangeli wrote:
> > From: Andrea Arcangeli <aarcange@...hat.com>
> > 
> > PG_buddy can be converted to _mapcount == -2. So the PG_compound_lock can be
> > added to page->flags without overflowing (because of the sparse section bits
> > increasing) with CONFIG_X86_PAE=y and CONFIG_X86_PAT=y. This also has to move
> > the memory hotplug code from _mapcount to lru.next to avoid any risk of
> > clashes. We can't use lru.next for PG_buddy removal, but memory hotplug can use
> > lru.next even more easily than the mapcount instead.
> > 
> 
> Does this make much of a difference? I confess I didn't read the patch closely
> because I didn't get the motivation.

The motivation is described in the first line. If I wouldn't remove
PG_buddy, introducing PG_compound_lock would overflow the 32bit build
with CONFIG_X86_PAE=y and CONFIG_X86_PAT=y. The bitflag easier to nuke
was PG_buddy.
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