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

[David Hildenbrand]

On 27.02.24 09:45, Lance Yang wrote:
> On Tue, Feb 27, 2024 at 4:33 PM David Hildenbrand <david@xxxxxxxxxx> 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@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.
>
> Sorry, I understood.
>
> Thanks for your clarification!

I'll post some kernel doc as reply to Barry's export patch to clarify that.

--
Cheers,

David / dhildenb