On Wed, Feb 5, 2020 at 1:00 AM Wang, Wei W <wei.w.wang@intel.com> wrote:

> 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.
>

Isn't the hint only useful during the *first* round?
After the first round if a page becomes free then we need to update the
copy at the migration destination, so freeing a page that previously had
contents should mark it dirty.


>
> Best,
> Wei
>