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

Ryan Roberts
2025-04-29T14:41:15Z

On 29/04/2025 15:29, 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.
>
> Something with XEN is really problematic here.
>

We are inside a lazy MMU region (arch_enter_lazy_mmu_mode()) at this point,
which I believe XEN uses. If a PTE was written then read back while in lazy mode
you could get a stale value.

See
https://lore.kernel.org/all/912c7a32-b39c-494f-a29c-4865cd92aeba@agordeev.local/
for an example bug.