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

[Lance Yang <ioworker0@gmail.com>]

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);

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.

Thanks,
Lance

>
> We know that the first PTE maps the folio. By incrementing the PFN using
> pte_next_pfn/pte_advance_pfn, we cannot suddenly get a lower PFN.
>
> So how would pte_advance_pfn(folio_start_pfn + X) suddenly give us a PFN
> lower than folio_start_pfn?
>
> Note that we are not really concerned about any kind of
> pte_advance_pfn() overflow that could generate PFN=0. I convinces myself
> that that that is something we don't have to worry about.
>
>
> [I also thought about getting rid of the pte_pfn(pte) >= folio_end_pfn
> and instead limiting end_ptep. But that requires more work before the
> loop and feels more like a micro-optimization.]
>
> >
> > Signed-off-by: Lance Yang <ioworker0@gmail.com>
> > ---
> >   mm/memory.c | 7 +++++--
> >   1 file changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/memory.c b/mm/memory.c
> > index 642b4f2be523..e5291d1e8c37 100644
> > --- a/mm/memory.c
> > +++ b/mm/memory.c
> > @@ -986,12 +986,15 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >               pte_t *start_ptep, pte_t pte, int max_nr, fpb_t flags,
> >               bool *any_writable)
> >   {
> > -     unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
> > +     unsigned long folio_start_pfn, folio_end_pfn;
> >       const pte_t *end_ptep = start_ptep + max_nr;
> >       pte_t expected_pte, *ptep;
> >       bool writable;
> >       int nr;
> >
> > +     folio_start_pfn = folio_pfn(folio);
> > +     folio_end_pfn = folio_start_pfn + folio_nr_pages(folio);
> > +
> >       if (any_writable)
> >               *any_writable = false;
> >
> > @@ -1015,7 +1018,7 @@ static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
> >                * corner cases the next PFN might fall into a different
> >                * folio.
> >                */
> > -             if (pte_pfn(pte) >= folio_end_pfn)
> > +             if (pte_pfn(pte) >= folio_end_pfn || pte_pfn(pte) < folio_start_pfn)
> >                       break;
> >
> >               if (any_writable)
>
> --
> Cheers,
>
> David / dhildenb
>