Re: [PATCH 1/1] mm: Fix folio_pte_batch() overcount with zero PTEs

Petr Vaněk
2025-04-29T14:45:57Z

On Tue, Apr 29, 2025 at 04:29:30PM +0200, David Hildenbrand wrote:
> On 29.04.25 16:22, Petr Vaněk wrote:
> > folio_pte_batch() could overcount the number of contiguous PTEs when
> > pte_advance_pfn() returns a zero-valued PTE and the following PTE in
> > memory also happens to be zero. The loop doesn't break in such a case
> > because pte_same() returns true, and the batch size is advanced by one
> > more than it should be.
> >
> > To fix this, bail out early if a non-present PTE is encountered,
> > preventing the invalid comparison.
> >
> > This issue started to appear after commit 10ebac4f95e7 ("mm/memory:
> > optimize unmap/zap with PTE-mapped THP") and was discovered via git
> > bisect.
> >
> > Fixes: 10ebac4f95e7 ("mm/memory: optimize unmap/zap with PTE-mapped THP")
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Petr Vaněk <arkamar@atlas.cz>
> > ---
> > mm/internal.h | 2 ++
> > 1 file changed, 2 insertions(+)
> >
> > diff --git a/mm/internal.h b/mm/internal.h
> > index e9695baa5922..c181fe2bac9d 100644
> > --- a/mm/internal.h
> > +++ b/mm/internal.h
> > @@ -279,6 +279,8 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> > dirty = !!pte_dirty(pte);
> > pte = __pte_batch_clear_ignored(pte, flags);
> >
> > + if (!pte_present(pte))
> > + break;
> > if (!pte_same(pte, expected_pte))
> > break;
>
> How could pte_same() suddenly match on a present and non-present PTE.

In the problematic case pte.pte == 0 and expected_pte.pte == 0 as well.
pte_same() returns a.pte == b.pte -> 0 == 0. Both are non-present PTEs.

> Something with XEN is really problematic here.
>
> --
> Cheers,
>
> David / dhildenb
>