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

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Aug 21, 2012 at 02:28:20PM -0300, Rafael Aquini wrote:
> On Tue, Aug 21, 2012 at 09:24:32AM -0700, Paul E. McKenney wrote:
> > On Tue, Aug 21, 2012 at 05:20:11PM +0200, Peter Zijlstra wrote:
> > > On Tue, 2012-08-21 at 09:47 -0300, Rafael Aquini wrote:
> > > > +       mapping = rcu_access_pointer(page->mapping);
> > > > +       if (mapping)
> > > > +               mapping = mapping->assoc_mapping; 
> > > 
> > > The comment near rcu_access_pointer() explicitly says:
> > > 
> > >  * Return the value of the specified RCU-protected pointer, but omit the
> > >  * smp_read_barrier_depends() and keep the ACCESS_ONCE().  This is useful
> > >  * when the value of this pointer is accessed, but the pointer is not
> > >  * dereferenced,
> > > 
> > > Yet you dereference the pointer... smells like fail to me.
> > 
> > Indeed!
> > 
> > This will break DEC Alpha.  In addition, if ->mapping can transition
> > from non-NULL to NULL, and if you used rcu_access_pointer() rather
> > than rcu_dereference() to avoid lockdep-RCU from yelling at you about
> > not either being in an RCU read-side critical section or holding an
> > update-side lock, you can see failures as follows:
> > 
> > 1.	CPU 0 runs the above code, picks up mapping, and finds it non-NULL.
> > 
> > 2.	CPU 0 is preempted or otherwise delayed.  (Keep in mind that
> > 	even disabling interrupts in a guest OS does not prevent the
> > 	host hypervisor from preempting!)
> > 
> > 3.	Some other CPU NULLs page->mapping.  Because CPU 0 isn't doing
> > 	anything to prevent it, this other CPU frees the memory.
> > 
> > 4.	CPU 0 resumes, and then accesses what is now the freelist.
> > 	Arbitrarily bad things start happening.
> > 
> > If you are in a read-side critical section, use rcu_dereference() instead
> > of rcu_access_pointer().  If you are holding an update-side lock, use
> > rcu_dereference_protected() and say what lock you are holding.  If you
> > are doing something else, please say what it is.
> > 
> > 							Thanx, Paul
> >
> Paul & Peter,
> 
> Thanks for looking into this stuff and providing me such valuable feedback, and
> RCU usage crashcourse.
> 
> I believe rcu_dereference_protected() is what I want/need here, since this code
> is always called for pages which we hold locked (PG_locked bit).

It would only help if we locked the page while updating the mapping,
as far as I can see we don't.

> So, it brings me
> to ask you if the following usage looks sane enough to fix the well pointed issue,
> or if it's another misuse of RCU API:
> 
> +       mapping = rcu_dereference_protecetd(page->mapping, PageLocked(page));
> +       if (mapping)
> +               mapping = mapping->assoc_mapping; 
>