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

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

On Wed, Aug 15, 2012 at 10:48:39AM +0100, Mel Gorman wrote:
> 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.

OK but checking it how? page->mapping == balloon_mapping does not scale to
multiple balloons, so I hoped we can switch to
page->mapping->flags & BALLOON_MAPPING or some such,
but this means we dereference it outside the lock ...

We will also need to add some API to set/clear mapping so that driver
does not need to poke in mm internals, but that's easy.

> -- 
> Mel Gorman
> SUSE Labs

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