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

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 11, 2021 at 09:44:07PM +0100, David Hildenbrand wrote:
> Let‘s take a look at init-on-free.
> 
> The guest zeroes a page and puts it onto a buddy freelist. Free page reporting code takes it off that list and reports it to the hypervisor. The hypervisor discards the physical page and tells the guest he‘s done processing the page. The guest re-places the page onto the free page list.
> 
> From that point on, the page can be re-allocated inside the guest and is assumed to be zero. On access, a fresh (zeroed) page is populated by the hypervisor. The guest won‘t re-zero the page, as it has the guarantee (from free page reporting) that the page remained zero.
> 
> Write-protecting the unpopulated page won‘t work as discussed.

IMHO no matter if it's init_on_alloc or init_on_free or both, as long as it's
inited in some way then it means the guest OS wrote to it.  Then wr-protect
will work..

MADV_DONTNEED during live snapshot seems to be a separate topic as you
mentioned in the other thread.  For that, I agree we'd better simply let
virtio_balloon_inhibited() to return true for live snapshot too just like
postcopy.

Thanks,

-- 
Peter Xu