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


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