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

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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@xxxxxxxxxx> 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





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