From mboxrd@z Thu Jan  1 00:00:00 1970
From: Ian Campbell <ian.campbell@citrix.com>
Subject: Re: [PATCH RFC 08/10] xen/balloon: implement migratepage
Date: Thu, 16 Oct 2014 10:28:03 +0100
Message-ID: <1413451683.2012.8.camel@citrix.com>
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To: David Vrabel <david.vrabel@citrix.com>
Cc: boris.ostrovsky@oracle.com, Wei Liu <wei.liu2@citrix.com>, stefano.stabellini@eu.citrix.com, xen-devel@lists.xen.org
List-Id: xen-devel@lists.xenproject.org

On Wed, 2014-10-15 at 17:16 +0100, David Vrabel wrote:
> On 15/10/14 16:54, Wei Liu wrote:
> > This patch replaces the xen_balloon_migratepage stub with actual
> > implementation.
> > 
> > It's implemented in two macro steps:
> > 1. populate old page with machine page and remove it from balloon driver
> > 2. give up machine page that backs new page and queue it to balloon
> >    driver
> > 
> > Old page is the page owned by balloon driver, new page is a page
> > isolated by core mm compaction thread.
> > 
> > The aforementioned steps swaps a ballooned out page with a usable page.
> > From guest's point of view, it de-fragments physical address space.
> > 
> > Signed-off-by: Wei Liu <wei.liu2@citrix.com>
> > ---
> >  drivers/xen/balloon.c |   94 +++++++++++++++++++++++++++++++++++++++++++++++--
> >  1 file changed, 92 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> > index 815e1d5..112190e 100644
> > --- a/drivers/xen/balloon.c
> > +++ b/drivers/xen/balloon.c
> > @@ -727,13 +727,103 @@ static struct notifier_block balloon_cpu_notifier = {
> >  
> >  static const struct address_space_operations xen_balloon_aops;
> >  #ifdef CONFIG_BALLOON_COMPACTION
> > +/*
> > + * xen_balloon_migratepage - perform the balloon page migration on behalf of
> > + *                           a compation thread.     (called under page lock)
> > + * @mapping: the page->mapping which will be assigned to the new migrated page
> > + * @newpage: page that will replace the isolated page after migration finishes
> > + * @page   : the isolated (old) page that is about to be migrated to newpage.
> > + * @mode   : compaction mode -- not used for balloon page migration.
> > + *
> > + * After a ballooned page gets isolated by compaction procedures, this
> > + * is the function that performs the page migration on behalf of a
> > + * compaction thread. The page migration for Xen balloon is done in
> > + * these two macro steps:
> > + *
> > + *  A. back @page with machine page
> > + *  B. release machine that backs @newpage
> 
> If the target if enforced and the guest is above the target.  This
> prevent any compaction.  You need an atomic swap operation.

XENMEM_exchange ought to be usable here I think.

Ian.