Re: [PATCH v9 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration

[Michael S. Tsirkin]

On Thu, Apr 13, 2017 at 01:44:11PM -0700, Matthew Wilcox wrote:
> On Thu, Apr 13, 2017 at 05:35:03PM +0800, Wei Wang wrote:
> > 2) transfer the guest unused pages to the host so that they
> > can be skipped to migrate in live migration.
> 
> I don't understand this second bit.  You leave the pages on the free list,
> and tell the host they're free.  What's preventing somebody else from
> allocating them and using them for something?  Is the guest semi-frozen
> at this point with just enough of it running to ask the balloon driver
> to do things?

There's missing documentation here.

The way things actually work is host sends to guest
a request for unused pages and then write-protects all memory.

So guest isn't frozen but any changes will be detected by host.