Re: [PATCH v13 0/5] UFFD write-tracking migration/snapshots

[Andrey Gruzdev]

On 24.02.2021 19:54, David Hildenbrand wrote:
> > > > Anyhow I'm quite sure that adding global MISSING handler for 
> > > > snapshotting
> > > > is too heavy and not really needed.
> > > UFFDIO_ZEROCOPY installs a zero pfn and that should be all of it.  
> > > There'll
> > > definitely be overhead, but it may not be that huge as imagined.  
> > > Live snapshot
> > > is great in that we have point-in-time image of guest without 
> > > stopping the
> > > guest, so taking slightly longer time won't be a huge loss to us too.
> > >
> > > Actually we can also think of other ways to work around it. One way 
> > > is we can
> > > pre-fault all guest pages before wr-protect.  Note that we don't 
> > > need to write
> > > to the guest page because read would suffice, since uffd-wp would 
> > > also work
> > > with zero pfn.  It's just that this workaround won't help on saving 
> > > snapshot
> > > disk space, but it seems working.  It would be great if you have other
> > > workarounds, maybe as you said UFFDIO_ZEROCOPY is not the only route.
> > >
> > > Thanks,
> > Just to add: one of the good options is too keep track of 
> > virtio-baloon discarded pages and
> > pre-fault them before migration starts. What do you think?
>
> Just pre-fault everything and inhibit the balloon. That should work.
Yep.

--
Andrey Gruzdev, Principal Engineer
Virtuozzo GmbH  +7-903-247-6397
                virtuzzo.com