Re: [PATCH v7 1/4] mm: introduce compaction and migration for virtio ballooned pages

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

On Tue, Aug 14, 2012 at 02:44:05PM -0300, Rafael Aquini wrote:
> On Mon, Aug 13, 2012 at 11:26:19AM +0300, Michael S. Tsirkin wrote:
> > > +static inline bool movable_balloon_page(struct page *page)
> > > +{
> > > +	return (page->mapping && page->mapping == balloon_mapping);
> > 
> > I am guessing this needs smp_read_barrier_depends, and maybe
> > ACCESS_ONCE ...
> > 
> 
> I'm curious about your guessing here. Could you ellaborate it further, please?
> 
> 
> > > +#else
> > > +static inline bool isolate_balloon_page(struct page *page) { return false; }
> > > +static inline void putback_balloon_page(struct page *page) { return false; }
> > > +static inline bool movable_balloon_page(struct page *page) { return false; }
> > > +#endif /* (VIRTIO_BALLOON || VIRTIO_BALLOON_MODULE) && CONFIG_COMPACTION */
> > > +
> > 
> > This does mean that only one type of balloon is useable at a time.
> > I wonder whether using a flag in address_space structure instead
> > is possible ...
> 
> This means we are only introducing this feature for virtio_balloon by now.
> Despite the flagging address_space stuff is something we surely can look in the
> future, I quite didn't get how we could be using two different types of balloon
> devices at the same time for the same system. Could you ellaborate it a little
> more, please?
> 

E.g. kvm can emulate hyperv so it could in theory have hyperv balloon.
This is mm stuff it is best not to tie it to specific drivers.

> > > +/* __isolate_lru_page() counterpart for a ballooned page */
> > > +bool isolate_balloon_page(struct page *page)
> > > +{
> > > +	if (WARN_ON(!movable_balloon_page(page)))
> > 
> > Looks like this actually can happen if the page is leaked
> > between previous movable_balloon_page and here.
> > 
> > > +		return false;
> 
> Yes, it surely can happen, and it does not harm to catch it here, print a warn and
> return.

If it is legal, why warn? For that matter why test here at all?

> While testing it, I wasn't lucky to see this small window opening, though.