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. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>