Re: [PATCH 1/1] mm/memory: Fix boundary check for next PFN in folio_pte_batch()

[David Hildenbrand <david@redhat.com>]

On 27.02.24 09:23, Lance Yang wrote:
> Hey David,
> 
> Thanks for taking time to review!
> 
> On Tue, Feb 27, 2024 at 3:30 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 27.02.24 08:04, Lance Yang wrote:
>>> Previously, in folio_pte_batch(), only the upper boundary of the
>>> folio was checked using '>=' for comparison. This led to
>>> incorrect behavior when the next PFN exceeded the lower boundary
>>> of the folio, especially in corner cases where the next PFN might
>>> fall into a different folio.
>>
>> Which commit does this fix?
>>
>> The introducing commit (f8d937761d65c87e9987b88ea7beb7bddc333a0e) is
>> already in mm-stable, so we would need a Fixes: tag. Unless, Ryan's
>> changes introduced a problem.
>>
>> BUT
>>
>> I don't see what is broken. :)
>>
>> Can you please give an example/reproducer?
> 
> For example1:
> 
> PTE0 is present for large folio1.
> PTE1 is present for large folio1.
> PTE2 is present for large folio1.
> PTE3 is present for large folio1.
> 
> folio_nr_pages(folio1) is 4.
> folio_nr_pages(folio2) is 4.
> 
> pte = *start_ptep = PTE0;
> max_nr = folio_nr_pages(folio2);
> 
> If folio_pfn(folio1) < folio_pfn(folio2),
> the return value of folio_pte_batch(folio2, start_ptep, pte, max_nr)
> will be 4(Actually it should be 0).
> 
> For example2:
> 
> PTE0 is present for large folio2.
> PTE1 is present for large folio1.
> PTE2 is present for large folio1.
> PTE3 is present for large folio1.
> 
> folio_nr_pages(folio1) is 4.
> folio_nr_pages(folio2) is 4.
> 
> pte = *start_ptep = PTE0;
> max_nr = folio_nr_pages(folio1);
> 

In both cases, start_ptep does not map the folio.

It's a BUG in your caller unless I am missing something important.


> If max_nr=4, the return value of folio_pte_batch(folio1, start_ptep,
> pte, max_nr)
> will be 1(Actually it should be 0).
> 
> folio_pte_batch() will soon be exported, and IMO, these corner cases may need
> to be handled.

No, you should fix your caller.

The function cannot possibly do something reasonable if start_ptep does 
not map the folio.

nr = pte_batch_hint(start_ptep, pte);
...
ptep = start_ptep + nr; /* nr is >= 1 */
...
return min(ptep - start_ptep, max_nr); /* will return something > 0 */

Which would return > 0 for something that does not map that folio.


I was trying to avoid official kernel docs for this internal helper, 
maybe we have to improve it now.

-- 
Cheers,

David / dhildenb