Re: [PATCH v1 0/3] migration: Fixes to the 'background-snapshot' code

[Peter Xu <peterx@redhat.com>]

On Wed, Mar 24, 2021 at 11:09:27AM +0300, Andrey Gruzdev wrote:
> > I'm also looking into introducing UFFD_FEATURE_WP_UNALLOCATED so as to
> > wr-protect page holes too for a uffd-wp region when the feature bit is set.
> > With that feature we should be able to avoid pre-fault as what we do in the
> > last patch of this series.  However even if that can work out, we'll still need
> > this for old kernel anyways.
> 
> I'm curious this new feature is based on adding wr-protection at the level of VMAs,
> so we won't miss write faults for missing pages?

I think we can do it with multiple ways.

The most efficient one would be wr-protect the range during uffd-wp
registration, so as you said it'll be per-vma attribute.  However that'll
change the general semantics of uffd-wp as normally we need registration and
explicit wr-protect.  Then it'll still be pte-based for faulted in pages (the
ones we wr-protected during registration will still be), however for the rest
it'll become vma-based.  It's indeed a bit confusing.

The other way is we can fault in zero page during UFFDIO_WRITEPROTECT.  However
that's less efficient, since it's close to pre-fault on read but it's just
slightly more cleaner than doing it in userspace.  When I rethink about this it
may not worth it to do in kernel if userspace can achieve things similar.

So let's stick with current solution; that idea may need more thoughts..

Thanks,

-- 
Peter Xu