Re: [PATCH v8 1/5] mm: introduce a common interface for balloon pages mobility

[Rafael Aquini <aquini@redhat.com>]

On Tue, Aug 21, 2012 at 04:52:23PM +0300, Michael S. Tsirkin wrote:
> > + * address_space_operations utilized methods for ballooned pages:
> > + *   .migratepage    - used to perform balloon's page migration (as is)
> > + *   .launder_page   - used to isolate a page from balloon's page list
> > + *   .freepage       - used to reinsert an isolated page to balloon's page list
> > + */
> 
> It would be a good idea to document the assumptions here.
> Looks like .launder_page and .freepage are called in rcu critical
> section.
> But migratepage isn't - why is that safe?
> 

The migratepage callback for virtio_balloon can sleep, and IIUC we cannot sleep
within a RCU critical section. 

Also, The migratepage callback is called at inner migration's circle function
move_to_new_page(), and I don't think embedding it in a RCU critical section
would be a good idea, for the same understanding aforementioned.