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

--
MST


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