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 10:19:31PM -0300, Rafael Aquini wrote:
> On Wed, Aug 22, 2012 at 03:07:41AM +0300, Michael S. Tsirkin wrote:
> > On Tue, Aug 21, 2012 at 05:45:56PM -0300, Rafael Aquini wrote:
> > > On Tue, Aug 21, 2012 at 10:30:31PM +0300, Michael S. Tsirkin wrote:
> > > > On Tue, Aug 21, 2012 at 04:23:58PM -0300, Rafael Aquini wrote:
> > > > > On Tue, Aug 21, 2012 at 10:13:30PM +0300, Michael S. Tsirkin wrote:
> > > > > > > 
> > > > > > > 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.
> > > > > >
> > > > > 
> > > > > But we can do it. In fact, by doing it (locking the page) we can easily avoid
> > > > > the nasty race balloon_isolate_page / leak_balloon, in a much simpler way, IMHO.
> > > > 
> > > > Absolutely. Further, we should look hard at whether most RCU uses
> > > > in this patchset can be replaced with page lock.
> > > >
> > > 
> > > Yeah, In fact, by testing/grabbing the page lock at leak_balloon() even the
> > > module unload X migration / putback race seems to fade away, since migration
> > > code holds the page locked all the way.
> > > And that seems a quite easy task to be accomplished:
> > > 
> > > ....
> > > @@ -169,21 +197,61 @@ static void leak_balloon(struct virtio_balloon *vb, size_t
> > > num)
> > >         /* We can only do one array worth at a time. */
> > >         num = min(num, ARRAY_SIZE(vb->pfns));
> > > 
> > > +       mutex_lock(&vb->balloon_lock);
> > >         for (vb->num_pfns = 0; vb->num_pfns < num;
> > >              vb->num_pfns += VIRTIO_BALLOON_PAGES_PER_PAGE) {
> > > +               spin_lock(&vb->pages_lock);
> > > +               /*
> > > +                * 'virtballoon_isolatepage()' can drain vb->pages list
> > > +                * making us to stumble across a _temporarily_ empty list.
> > 
> > This still worries me. If this happens we do not
> > lock the page so module can go away?
> > if not need to document why.
> >
> The module won't unload unless it leaks all its pages. If we hit that test that
> worries you, leak_balloon() will get back to its caller -- remove_common(), and
> it will kept looping at:
> 
>         /* There might be pages left in the balloon: free them. */
>         while (vb->num_pages)
>                 leak_balloon(vb, vb->num_pages);
> 
> This is true because we do not mess with vb->num_pages while isolating/migrating
> balloon pages, so the module will only unload when all isolated pages get back
> to vb->pages_list and leak_balloon() reap them appropriatelly. As we will be
> doing isolation/migration/putback steps under 'page lock' that race is gone.


Hmm, so this will busy wait which is unelegant.
We need some event IMO.
Also, reading num_pages without a lock here
which seems wrong.

A similar concern applies to normal leaking
of the balloon: here we might leak less than
required, then wait for the next config change
event.

How about we signal config_change
event when pages are back to pages_list?

-- 
MST

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