Re: [PATCH v9 3/5] virtio_balloon: introduce migration primitives to balloon pages

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

On Mon, Aug 27, 2012 at 04:47:13PM -0300, Rafael Aquini wrote:
> On Sun, Aug 26, 2012 at 10:42:44AM +0300, Michael S. Tsirkin wrote:
> > 
> > Reading two atomics and doing math? Result can even be negative.
> > I did not look at use closely but it looks suspicious.
> Doc on atomic_read says:
> "
> The read is atomic in that the return value is guaranteed to be one of the
> values initialized or modified with the interface operations if a proper
> implicit or explicit memory barrier is used after possible runtime
> initialization by any other thread and the value is modified only with the
> interface operations.
> "
> 
> There's no runtime init by other thread than balloon's itself at device register,
> and the operations (inc, dec) are made by the proper interface operations
> only when protected by the spinlock pages_lock. It does not look suspicious, IMHO.

Any use of multiple atomics is suspicious.
Please just avoid it if you can. What's wrong with locking?

> I'm failing to see how it could become a negative on that case, since you cannot
> isolate more pages than what was previoulsy inflated to balloon's list.

There is no order guarantee. So in
A - B you can read B long after both A and B has been incremented.
Maybe it is safe in this case but it needs careful documentation
to explain how ordering works. Much easier to keep it all simple.

> 
> > It's already the case everywhere except __wait_on_isolated_pages,
> > so just fix that, and then we can keep using int instead of atomics.
> > 
> Sorry, I quite didn't get you here. fix what?

It's in the text you removed above. Access values under lock.

>  
> > That's 1K on stack - and can become more if we increase
> > VIRTIO_BALLOON_ARRAY_PFNS_MAX.  Probably too much - this is the reason
> > we use vb->pfns.
> >
> If we want to use vb->pfns we'll have to make leak_balloon mutual exclusive with
> page migration (as it was before), but that will inevictably bring us back to
> the discussion on breaking the loop when isolated pages make leak_balloon find
> less pages than it wants to release at each leak round.
> 

I don't think this is an issue. The issue was busy waiting in that case.

-- 
MST

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