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 12:16:51PM +0100, Mel Gorman wrote:
> On Wed, Aug 15, 2012 at 01:01:08PM +0300, Michael S. Tsirkin wrote:
> > 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,
> 
> I was thinking of exactly that page->mapping == balloon_mapping check. As I
> do not know how many active balloon drivers there might be I cannot guess
> in advance how much of a scalability problem it will be.

Not at all sure multiple drivers are worth supporting, but multiple
*devices* is I think worth supporting, if for no other reason than that
they can work today. For that, we need a device pointer which Rafael
wants to put into the mapping, this means multiple balloon mappings.


> > so I hoped we can switch to
> > page->mapping->flags & BALLOON_MAPPING or some such,
> > but this means we dereference it outside the lock ...
> > 
> 
> That also sounded like future stuff to me that would be justified with
> profiling if necessary. Personally I would have started with the spinlock
> and a simple check and moved to RCU later when either scalability was a
> problem or it was found there was a need to stabilise whether a page was
> a balloon page or not outside a spinlock.
> 
> This is not a NAK to the idea and I'm not objecting to RCU being used now
> if that is what is really desired. I just suspect it's making the series
> more complex than it needs to be right now.
> 
> -- 
> Mel Gorman
> SUSE Labs

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