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.