linux-kernel - Re: [PATCH 04/15] mm: compaction: introduce isolate_freepages

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Message-ID: <20120130115548.GH25268@csn.ul.ie>
Date:	Mon, 30 Jan 2012 11:55:48 +0000
From:	Mel Gorman <mel@....ul.ie>
To:	Marek Szyprowski <m.szyprowski@...sung.com>
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	linux-media@...r.kernel.org, linux-mm@...ck.org,
	linaro-mm-sig@...ts.linaro.org,
	Michal Nazarewicz <mina86@...a86.com>,
	Kyungmin Park <kyungmin.park@...sung.com>,
	Russell King <linux@....linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Daniel Walker <dwalker@...eaurora.org>,
	Arnd Bergmann <arnd@...db.de>,
	Jesse Barker <jesse.barker@...aro.org>,
	Jonathan Corbet <corbet@....net>,
	Shariq Hasnain <shariq.hasnain@...aro.org>,
	Chunsang Jeong <chunsang.jeong@...aro.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Benjamin Gaignard <benjamin.gaignard@...aro.org>
Subject: Re: [PATCH 04/15] mm: compaction: introduce isolate_freepages_range()

On Mon, Jan 30, 2012 at 11:48:20AM +0000, Mel Gorman wrote:
> > +		if (!zone)
> > +			zone = page_zone(pfn_to_page(pfn));
> > +		else if (zone != page_zone(pfn_to_page(pfn)))
> > +			break;
> > +
> 
> So what you are checking for here is if you straddle zones.
> You could just initialise zone outside of the for loop. You can
> then check outside the loop if end_pfn is in a different zone to
> start_pfn. If it is, either adjust end_pfn accordingly or bail the
> entire operation avoiding the need for release_freepages() later. This
> will be a little cheaper.
> 

Whoops, silly me! You are watching for overlapping zones which can
happen in some rare configurations and for that checking page_zone()
like this is necessary. You can still initialise zone outside the loop
but the page_zone() check is still necessary.

My bad.

-- 
Mel Gorman
SUSE Labs
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