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

[David Hildenbrand <david@redhat.com>]

>>> 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.

-- 
Thanks,

David / dhildenb