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

[David Hildenbrand <david@redhat.com>]

On 29.04.25 16:41, Ryan Roberts wrote:
> 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.

So if we cannot trust ptep_get() output, then, ... how could we trust 
anything here and ever possibly batch?

-- 
Cheers,

David / dhildenb