linux-kernel - Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

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Message-ID: <20120815094839.GJ4052@csn.ul.ie>
Date:	Wed, 15 Aug 2012 10:48:39 +0100
From:	Mel Gorman <mel@....ul.ie>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	Rafael Aquini <aquini@...hat.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	Rusty Russell <rusty@...tcorp.com.au>,
	Rik van Riel <riel@...hat.com>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>
Subject: Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to
 balloon pages

On Wed, Aug 15, 2012 at 12:25:28PM +0300, Michael S. Tsirkin wrote:
> On Wed, Aug 15, 2012 at 10:05:28AM +0100, Mel Gorman wrote:
> > On Tue, Aug 14, 2012 at 05:11:13PM -0300, Rafael Aquini wrote:
> > > On Tue, Aug 14, 2012 at 10:51:39PM +0300, Michael S. Tsirkin wrote:
> > > > What I think you should do is use rcu for access.
> > > > And here sync rcu before freeing.
> > > > Maybe an overkill but at least a documented synchronization
> > > > primitive, and it is very light weight.
> > > > 
> > > 
> > > I liked your suggestion on barriers, as well.
> > > 
> > 
> > I have not thought about this as deeply as I shouold but is simply rechecking
> > the mapping under the pages_lock to make sure the page is still a balloon
> > page an option? i.e. use pages_lock to stabilise page->mapping.
> 
> To clarify, are you concerned about cost of rcu_read_lock
> for non balloon pages?
> 

Not as such, but given the choice between introducing RCU locking and
rechecking page->mapping under a spinlock I would choose the latter as it
is more straight-forward.

-- 
Mel Gorman
SUSE Labs
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