On 03/20/2013 03:06 PM, Michael S. Tsirkin wrote:

No, not just ballooning. Overcommit (i.e. cgroups).

Anytime cgroups kicks out a page (or anytime the balloon kicks in),
the page would become unmapped.

OK but we still need to send that page to remote.
It's in swap but has guest data in there, you can't
just ignore it.

Yes, absolutely: https://www.kernel.org/doc/Documentation/vm/pagemap.txt

The pagemap will tell you that.

In fact the pagemap ideally would *only* be used for the 1st migration round.

The rest of them would depend exclusively on the dirty bitmap as they do.

Basically, we could use the pagemap as first-time "hint" for the bulk of
the memory that costs the most to transmit.