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

[David Hildenbrand <david@redhat.com>]

On 29.04.25 20:33, Petr Vaněk wrote:
> On Tue, Apr 29, 2025 at 05:45:53PM +0200, David Hildenbrand wrote:
>> 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 issue is not happening in __pte_batch_clear_ignored but later in
> following line:
> 
>    expected_pte = pte_advance_pfn(expected_pte, nr);
> 
> The issue seems to be in __pte function which converts PTE value to
> pte_t in pte_advance_pfn, because warnings disappears when I change the
> line to
> 
>    expected_pte = (pte_t){ .pte = pte_val(expected_pte) + (nr << PFN_PTE_SHIFT) };
> 
> The kernel probably uses __pte function from
> arch/x86/include/asm/paravirt.h because it is configured with
> CONFIG_PARAVIRT=y:
> 
>    static inline pte_t __pte(pteval_t val)
>    {
>    	return (pte_t) { PVOP_ALT_CALLEE1(pteval_t, mmu.make_pte, val,
>    					  "mov %%rdi, %%rax", ALT_NOT_XEN) };
>    }
> 
> I guess it might cause this weird magic, but I need more time to
> understand what it does :)

What XEN does with basic primitives that convert between pteval and 
pte_t is beyond horrible.

How come set_ptes() that uses pte_next_pfn()->pte_advance_pfn() does not 
run into this?

Is it only a problem if we exceed a certain pfn?

-- 
Cheers,

David / dhildenb