linux-kernel - Re: [PATCH v9 1/5] mm: introduce a common interface for balloon pages mobility

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Date:	Tue, 28 Aug 2012 18:23:12 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Rafael Aquini <aquini@...hat.com>
Cc:	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>, Mel Gorman <mel@....ul.ie>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Minchan Kim <minchan@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH v9 1/5] mm: introduce a common interface for balloon
 pages mobility

On Mon, Aug 27, 2012 at 05:28:35PM -0300, Rafael Aquini wrote:
> On Sun, Aug 26, 2012 at 10:55:58AM +0300, Michael S. Tsirkin wrote:
> > On Sat, Aug 25, 2012 at 02:24:56AM -0300, 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 a common interface to help a balloon driver on
> > > making its page set movable to compaction, and thus allowing the system
> > > to better leverage the compation efforts on memory defragmentation.
> > > 
> > > Signed-off-by: Rafael Aquini <aquini@...hat.com>
> > 
> > Tons of rcu uses but not sync in sight. This looks suspicious.
> 
> There's no critical section marked with rcu_read_lock/rcu_read_unlock. that's
> why there's no call for sync anywhere. As we are behaving mostly as updaters,
> the hole rcu usage is awkward and it's placed basically to enforce the proper
> order. To avoid hurting the RCU API usage with this awk approach I'll drop it
> for the next series submission (it will use barriers instead).

If everything is under page lock, barriers are likely not required.
If not, they might not be sufficient.

-- 
MST
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