On Tue, Aug 21, 2012 at 06:41:42PM +0300, Michael S. Tsirkin wrote: > 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. > No, I need to dereference page->mapping to check ->mapping flags here, before returning. Remember this function is used at MM's compaction/migration inner circles to identify ballooned pages and decide what's the next step. This function is doing the right thing, IMHO. Also, looking at how compaction/migration work, we verify the only critical path for this function is the page isolation step. The other steps (migration and putback) perform their work on private lists previouly isolated from a given source. So, we just need to make sure that the isolation part does not screw things up by isolating pages that balloon driver is about to release. That's why there are so many checkpoints down the page isolation path assuring we really are isolating a balloon page. -- 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>