Re: [PATCH v11 4/7] mm: introduce compaction and migration for ballooned pages

[Andrew Morton <akpm@linux-foundation.org>]

On Fri, 9 Nov 2012 12:16:02 +0000
Mel Gorman <mel@csn.ul.ie> wrote:

> On Wed, Nov 07, 2012 at 01:05:51AM -0200, Rafael Aquini wrote:
> > Memory fragmentation introduced by ballooning might reduce significantly
> > the number of 2MB contiguous memory blocks that can be used within a guest,
> > thus imposing performance penalties associated with the reduced number of
> > transparent huge pages that could be used by the guest workload.
> > 
> > This patch introduces the helper functions as well as the necessary changes
> > to teach compaction and migration bits how to cope with pages which are
> > part of a guest memory balloon, in order to make them movable by memory
> > compaction procedures.
> > 
>
> ...
>
> > --- a/mm/compaction.c
> > +++ b/mm/compaction.c
> > @@ -14,6 +14,7 @@
> >  #include <linux/backing-dev.h>
> >  #include <linux/sysctl.h>
> >  #include <linux/sysfs.h>
> > +#include <linux/balloon_compaction.h>
> >  #include "internal.h"
> >  
> >  #if defined CONFIG_COMPACTION || defined CONFIG_CMA
> > @@ -565,9 +566,24 @@ isolate_migratepages_range(struct zone *zone, struct compact_control *cc,
> >  			goto next_pageblock;
> >  		}
> >  
> > -		/* Check may be lockless but that's ok as we recheck later */
> > -		if (!PageLRU(page))
> > +		/*
> > +		 * Check may be lockless but that's ok as we recheck later.
> > +		 * It's possible to migrate LRU pages and balloon pages
> > +		 * Skip any other type of page
> > +		 */
> > +		if (!PageLRU(page)) {
> > +			if (unlikely(balloon_page_movable(page))) {
> 
> Because it's lockless, it really seems that the barrier stuck down there
> is unnecessary. At worst you get a temporarily incorrect answer that you
> recheck later under page lock in balloon_page_isolate.

What happened with this?

Also: what barrier?