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

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Message-ID: <2025429144547-aBDmGzJBQc9RMBj--arkamar@atlas.cz>
Date: Tue, 29 Apr 2025 16:45:47 +0200
From: Petr Vaněk <arkamar@...as.cz>
To: David Hildenbrand <david@...hat.com>
Cc: linux-kernel@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Ryan Roberts <ryan.roberts@....com>, linux-mm@...ck.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/1] mm: Fix folio_pte_batch() overcount with zero PTEs

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@...r.kernel.org
> > Signed-off-by: Petr Vaněk <arkamar@...as.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
>