Re: [PATCHv4 2/3] mm/gup: fix omission of check on FOLL_LONGTERM in gup fast path

[Pingfan Liu <kernelfans@gmail.com>]

Cc Mike, David, who is an expert of hugetlb and thp

On Fri, Jun 14, 2019 at 5:37 AM Ira Weiny <ira.weiny@intel.com> wrote:
>
> On Thu, Jun 13, 2019 at 06:45:01PM +0800, Pingfan Liu wrote:
> > FOLL_LONGTERM suggests a pin which is going to be given to hardware and
> > can't move. It would truncate CMA permanently and should be excluded.
> >
> > FOLL_LONGTERM has already been checked in the slow path, but not checked in
> > the fast path, which means a possible leak of CMA page to longterm pinned
> > requirement through this crack.
> >
> > Place a check in gup_pte_range() in the fast path.
> >
> > Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> > Cc: Ira Weiny <ira.weiny@intel.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Mike Rapoport <rppt@linux.ibm.com>
> > Cc: Dan Williams <dan.j.williams@intel.com>
> > Cc: Matthew Wilcox <willy@infradead.org>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > Cc: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
> > Cc: Keith Busch <keith.busch@intel.com>
> > Cc: Christoph Hellwig <hch@infradead.org>
> > Cc: Shuah Khan <shuah@kernel.org>
> > Cc: linux-kernel@vger.kernel.org
> > ---
> >  mm/gup.c | 26 ++++++++++++++++++++++++++
> >  1 file changed, 26 insertions(+)
> >
> > diff --git a/mm/gup.c b/mm/gup.c
> > index 766ae54..de1b03f 100644
> > --- a/mm/gup.c
> > +++ b/mm/gup.c
> > @@ -1757,6 +1757,14 @@ static int gup_pte_range(pmd_t pmd, unsigned long addr, unsigned long end,
> >               VM_BUG_ON(!pfn_valid(pte_pfn(pte)));
> >               page = pte_page(pte);
> >
> > +             /*
> > +              * FOLL_LONGTERM suggests a pin given to hardware. Prevent it
> > +              * from truncating CMA area
> > +              */
> > +             if (unlikely(flags & FOLL_LONGTERM) &&
> > +                     is_migrate_cma_page(page))
> > +                     goto pte_unmap;
> > +
> >               head = try_get_compound_head(page, 1);
> >               if (!head)
> >                       goto pte_unmap;
> > @@ -1900,6 +1908,12 @@ static int gup_huge_pmd(pmd_t orig, pmd_t *pmdp, unsigned long addr,
> >               refs++;
> >       } while (addr += PAGE_SIZE, addr != end);
> >
> > +     if (unlikely(flags & FOLL_LONGTERM) &&
> > +             is_migrate_cma_page(page)) {
> > +             *nr -= refs;
> > +             return 0;
> > +     }
> > +
>
> Why can't we place this check before the while loop and skip subtracting the
> page count?
Yes, that will be better.

>
> Can is_migrate_cma_page() operate on any "subpage" of a compound page?
For gigantic page, __alloc_gigantic_page() allocate from
MIGRATE_MOVABLE pageblock. For page order < MAX_ORDER, pages are
allocated from either free_list[MIGRATE_MOVABLE] or
free_list[MIGRATE_CMA]. So all subpage have the same migrate type.

Thanks,
  Pingfan
>
> Here this calls is_magrate_cma_page() on the tail page of the compound page.
>
> I'm not an expert on compound pages nor cma handling so is this ok?
>
> It seems like you need to call is_migrate_cma_page() on each page within the
> while loop?
>
> >       head = try_get_compound_head(pmd_page(orig), refs);
> >       if (!head) {
> >               *nr -= refs;
> > @@ -1941,6 +1955,12 @@ static int gup_huge_pud(pud_t orig, pud_t *pudp, unsigned long addr,
> >               refs++;
> >       } while (addr += PAGE_SIZE, addr != end);
> >
> > +     if (unlikely(flags & FOLL_LONGTERM) &&
> > +             is_migrate_cma_page(page)) {
> > +             *nr -= refs;
> > +             return 0;
> > +     }
> > +
>
> Same comment here.
>
> >       head = try_get_compound_head(pud_page(orig), refs);
> >       if (!head) {
> >               *nr -= refs;
> > @@ -1978,6 +1998,12 @@ static int gup_huge_pgd(pgd_t orig, pgd_t *pgdp, unsigned long addr,
> >               refs++;
> >       } while (addr += PAGE_SIZE, addr != end);
> >
> > +     if (unlikely(flags & FOLL_LONGTERM) &&
> > +             is_migrate_cma_page(page)) {
> > +             *nr -= refs;
> > +             return 0;
> > +     }
> > +
>
> And here.
>
> Ira
>
> >       head = try_get_compound_head(pgd_page(orig), refs);
> >       if (!head) {
> >               *nr -= refs;
> > --
> > 2.7.5
> >