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
