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

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Date:   Fri, 14 Apr 2017 04:50:48 +0300
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Wei Wang <wei.w.wang@...el.com>, virtio-dev@...ts.oasis-open.org,
        linux-kernel@...r.kernel.org, qemu-devel@...gnu.org,
        virtualization@...ts.linux-foundation.org, kvm@...r.kernel.org,
        linux-mm@...ck.org, david@...hat.com, dave.hansen@...el.com,
        cornelia.huck@...ibm.com, akpm@...ux-foundation.org,
        mgorman@...hsingularity.net, aarcange@...hat.com,
        amit.shah@...hat.com, pbonzini@...hat.com,
        liliang.opensource@...il.com
Subject: Re: [PATCH v9 0/5] Extend virtio-balloon for fast (de)inflating &
 fast live migration

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.