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

On Mon, Jan 30, 2012 at 11:48:20AM +0000, Mel Gorman wrote:

quoted

+		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