Re: [PATCH v3 1/2] mm/migrate: Fix read-only page got writable when recover pte

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 15 Nov 2022 19:17:43 +0100 David Hildenbrand <david@redhat.com> 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.