Re: [PATCH v7 2/4] virtio_balloon: introduce migration primitives to balloon pages

[Rafael Aquini <aquini@redhat.com>]

On Tue, Aug 14, 2012 at 11:33:20AM +0300, Michael S. Tsirkin wrote:
> On Tue, Aug 14, 2012 at 09:29:49AM +0930, Rusty Russell wrote:
> > On Mon, 13 Aug 2012 11:41:23 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > On Fri, Aug 10, 2012 at 02:55:15PM -0300, Rafael Aquini wrote:
> > > > +/*
> > > > + * Populate balloon_mapping->a_ops->freepage method to help compaction on
> > > > + * re-inserting an isolated page into the balloon page list.
> > > > + */
> > > > +void virtballoon_putbackpage(struct page *page)
> > > > +{
> > > > +	spin_lock(&pages_lock);
> > > > +	list_add(&page->lru, &vb_ptr->pages);
> > > > +	spin_unlock(&pages_lock);
> > > 
> > > Could the following race trigger:
> > > migration happens while module unloading is in progress,
> > > module goes away between here and when the function
> > > returns, then code for this function gets overwritten?
> > > If yes we need locking external to module to prevent this.
> > > Maybe add a spinlock to struct address_space?
> > 
> > The balloon module cannot be unloaded until it has leaked all its pages,
> > so I think this is safe:
> > 
> >         static void remove_common(struct virtio_balloon *vb)
> >         {
> >         	/* There might be pages left in the balloon: free them. */
> >         	while (vb->num_pages)
> >         		leak_balloon(vb, vb->num_pages);
> > 
> > Cheers,
> > Rusty.
> 
> I know I meant something else.
> Let me lay this out:
> 
> CPU1 executes:
> void virtballoon_putbackpage(struct page *page)
> {
> 	spin_lock(&pages_lock);
> 	list_add(&page->lru, &vb_ptr->pages);
> 	spin_unlock(&pages_lock);
> 
> 
> 		at this point CPU2 unloads module:
> 						leak_balloon
> 						......
> 
> 		next CPU2 loads another module so code memory gets overwritten
> 
> now CPU1 executes the next instruction:
> 
> }
> 
> which would normally return to function's caller,
> but it has been overwritten by CPU2 so we get corruption.
> 
> No?

At the point CPU2 is unloading the module, it will be kept looping at the
snippet Rusty pointed out because the isolation / migration steps do not mess
with 'vb->num_pages'. The driver will only unload after leaking the total amount
of balloon's inflated pages, which means (for this hypothetical case) CPU2 will
wait until CPU1 finishes the putaback procedure.