[PATCH 04/15] mm: compaction: introduce isolate_freepages_range()

[Mel Gorman]

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