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

[Lance Yang <ioworker0@gmail.com>]

On Tue, Feb 27, 2024 at 4:33 PM David Hildenbrand <david@redhat.com> wrote:
>
> 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.

Sorry, I understood.

Thanks for your clarification!
Lance

>
>
> > 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
>