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

[Wei Wang]

On 04/14/2017 09:50 AM, Michael S. Tsirkin wrote:
> 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.

Probably it's better to say " transfer the info about the guest unused pages
to the host so that the host gets a chance to skip the transfer of the 
unused
pages during live migration".

Best,
Wei