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:

quoted

quoted

+             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@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>