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

[David Hildenbrand <david@redhat.com>]

On 29.04.25 16:45, Petr Vaněk wrote:
> 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.

Observe that folio_pte_batch() was called *with a present pte*.

do_zap_pte_range()
	if (pte_present(ptent))
		zap_present_ptes()
			folio_pte_batch()

How can we end up with an expected_pte that is !present, if it is based 
on the provided pte that *is present* and we only used pte_advance_pfn() 
to advance the pfn?

-- 
Cheers,

David / dhildenb