RE: Balloon pressuring page cache

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On Wednesday, February 5, 2020 4:57 PM, David Hildenbrand wrote:
> >> Yes, I agree with you. Yet, I am thinking about one
> >> (unlikely?impossible?) scenario. Can you refresh my brain why that
> >> cannot happen (IOW, why we don't have to wait for the host to process
> >> the request)?
> >>
> >> 1. Guest allocates a page and sends it to the host.
> >> 2. Shrinker gets active and releases that page again.
> >> 3. Some user in the guest allocates and modifies that page. After
> >> that, it is done using that page for the next hour.
> >> 4. The host processes the request and clears the bit in the dirty bitmap.
> >> 5. The guest is being migrated by the host. The modified page is not
> >> being migrated.
> >
> > Whenever the guest modifies a page during migration, it will be
> > captured by the dirty logging and the hypervisor will send the dirtied the
> page in the following round.
> 
> Please explain why the steps I outlined don't apply esp. in the last round.
> Your general statement does not explain why this race can't happen.
> 

The guest is stopped in the last round, thus no page will be modified at that time.

Best,
Wei





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