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

[Peter Xu]

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