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

On Fri, 9 Nov 2012 12:16:02 +0000
Mel Gorman [off-list ref] wrote:

On Wed, Nov 07, 2012 at 01:05:51AM -0200, Rafael Aquini wrote:

quoted

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.

...

quoted

--- 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?