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

[David Hildenbrand <david@redhat.com>]

On 29.04.25 16:52, David Hildenbrand wrote:
> 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?

I've been staring at the code for too long and don't see the issue.

We even have

VM_WARN_ON_FOLIO(!pte_present(pte), folio);

So the initial pteval we got is present.

I don't see how

	nr = pte_batch_hint(start_ptep, pte);
	expected_pte = __pte_batch_clear_ignored(pte_advance_pfn(pte, nr), flags);

would suddenly result in !pte_present(expected_pte).

The really weird thing is that this has only been seen on XEN.

But even on XEN, a present pte should not suddenly get !present -- we're not
re-reading from ptep :/

-- 
Cheers,

David / dhildenb