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

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Message-ID: <58F033D0.7080101@intel.com>
Date:   Fri, 14 Apr 2017 10:28:32 +0800
From:   Wei Wang <wei.w.wang@...el.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Matthew Wilcox <willy@...radead.org>
CC:     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 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