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

["Michael S. Tsirkin" <mst@redhat.com>]

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@redhat.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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