Re: [Linaro-mm-sig] [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.