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

[Rafael Aquini]

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.
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.


> 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?

 
> 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.

 
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