On Tue, Aug 21, 2012 at 05:16:06PM +0200, Peter Zijlstra wrote: > On Tue, 2012-08-21 at 16:52 +0300, Michael S. Tsirkin wrote: > > > + rcu_read_lock(); > > > + mapping = rcu_dereference(page->mapping); > > > + if (mapping_balloon(mapping)) > > > + ret = true; > > > + rcu_read_unlock(); > > > > This looks suspicious: you drop rcu_read_unlock > > so can't page switch from balloon to non balloon? > > RCU read lock is a non-exclusive lock, it cannot avoid anything like > that. You are right, of course. So even keeping rcu_read_lock across both test and operation won't be enough - you need to make this function return the mapping and pass it to isolate_page/putback_page so that it is only dereferenced once. -- MST -- 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>