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

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