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

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

On Tue, Aug 21, 2012 at 04:34:39PM -0300, Rafael Aquini wrote:
> On Tue, Aug 21, 2012 at 10:16:12PM +0300, Michael S. Tsirkin wrote:
> > On Tue, Aug 21, 2012 at 02:55:03PM -0300, Rafael Aquini wrote:
> > > 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.
> > 
> > Yes but this means it is still exposed to the module unloading
> > races that RCU was supposed to fix.
> > So need to either rework that code so it won't sleep
> > or switch to some other synchronization.
> >
> Can you refactor tell_host() to not sleep? Or, can I get rid of calling it at
> virtballoon_migratepage()? If 'no' is the answer for both questions, that's the
> way that code has to remain, even if we find a way around to hack the
> migratepage callback and have it embedded into a RCU crit section.
> 
> That's why I believe once the balloon driver is commanded to unload, we must
> flag virtballoon_migratepage to skip it's work. By doing this, the thread
> performing memory compaction will have to recur to the 'putback' path which is
> RCU protected. (IMHO).
> 
> As the module will not uload utill it leaks all pages on its list, that unload
> race you pointed before will be covered.


It can not be: nothing callback does can prevent it from
running after module unload: you must have some synchronization
in the calling code.

-- 
MST