On Tue, 15 Nov 2022 19:17:43 +0100 David Hildenbrand <david@xxxxxxxxxx> wrote:
> On 14.11.22 01:04, Peter Xu wrote:
> > Ives van Hoorne from codesandbox.io reported an issue regarding possible
> > data loss of uffd-wp when applied to memfds on heavily loaded systems. The
> > symptom is some read page got data mismatch from the snapshot child VMs.
> >
> > Here I can also reproduce with a Rust reproducer that was provided by Ives
> > that keeps taking snapshot of a 256MB VM, on a 32G system when I initiate
> > 80 instances I can trigger the issues in ten minutes.
> >
> > It turns out that we got some pages write-through even if uffd-wp is
> > applied to the pte.
> >
> > The problem is, when removing migration entries, we didn't really worry
> > about write bit as long as we know it's not a write migration entry. That
> > may not be true, for some memory types (e.g. writable shmem) mk_pte can
> > return a pte with write bit set, then to recover the migration entry to its
> > original state we need to explicit wr-protect the pte or it'll has the
> > write bit set if it's a read migration entry. For uffd it can cause
> > write-through.
> >
> > The relevant code on uffd was introduced in the anon support, which is
> > commit f45ec5ff16a7 ("userfaultfd: wp: support swap and page migration",
> > 2020-04-07). However anon shouldn't suffer from this problem because anon
> > should already have the write bit cleared always, so that may not be a
> > proper Fixes target, while I'm adding the Fixes to be uffd shmem support.
> >
>
> ...
>
> > --- a/mm/migrate.c
> > +++ b/mm/migrate.c
> > @@ -213,8 +213,14 @@ static bool remove_migration_pte(struct folio *folio,
> > pte = pte_mkdirty(pte);
> > if (is_writable_migration_entry(entry))
> > pte = maybe_mkwrite(pte, vma);
> > - else if (pte_swp_uffd_wp(*pvmw.pte))
> > + else
> > + /* NOTE: mk_pte can have write bit set */
> > + pte = pte_wrprotect(pte);
> > +
> > + if (pte_swp_uffd_wp(*pvmw.pte)) {
> > + WARN_ON_ONCE(pte_write(pte));
Will this warnnig trigger in the scenario you and Ives have discovered?
> > pte = pte_mkuffd_wp(pte);
> > + }
> >
> > if (folio_test_anon(folio) && !is_readable_migration_entry(entry))
> > rmap_flags |= RMAP_EXCLUSIVE;
>
> As raised, I don't agree to this generic non-uffd-wp change without
> further, clear justification.
Pater, can you please work this further?
> I won't nack it, but I won't ack it either.
I wouldn't mind seeing a little code comment which explains why we're
doing this.
