Re: [PATCH 1/1] mm: mempolicy: fix mbind_range() && vma_adjust() interaction

[Oleg Nesterov <oleg@redhat.com>]

On 07/08, KOSAKI Motohiro wrote:
>
> On Mon, Jul 8, 2013 at 2:05 PM, Oleg Nesterov <oleg@redhat.com> wrote:
> > vma_adjust() does vma_set_policy(vma, vma_policy(next)) and this
> > is doubly wrong:
> >
> > 1. This leaks vma->vm_policy if it is not NULL and not equal to
> >    next->vm_policy.
> >
> >    This can happen if vma_merge() expands "area", not prev (case 8).
> >
> > 2. This sets the wrong policy if vma_merge() joins prev and area,
> >    area is the vma the caller needs to update and it still has the
> >    old policy.
> >
> > Revert 1444f92c "mm: merging memory blocks resets mempolicy" which
> > introduced these problems.
>
> Yes, I believe 1444f92c is wrong and should be reverted.

Yes, but the problem it tried to solve is real, just we can't rely
on vma_adjust().

> > Change mbind_range() to recheck mpol_equal() after vma_merge() to
> > fix the problem 1444f92c tried to address.
> >
> > Signed-off-by: Oleg Nesterov <oleg@redhat.com>
> > Cc: <stable@vger.kernel.org>
> > ---
> >  mm/mempolicy.c |    6 +++++-
> >  mm/mmap.c      |    2 +-
> >  2 files changed, 6 insertions(+), 2 deletions(-)
> >
> > diff --git a/mm/mempolicy.c b/mm/mempolicy.c
> > index 7431001..4baf12e 100644
> > --- a/mm/mempolicy.c
> > +++ b/mm/mempolicy.c
> > @@ -732,7 +732,10 @@ static int mbind_range(struct mm_struct *mm, unsigned long start,
> >                 if (prev) {
> >                         vma = prev;
> >                         next = vma->vm_next;
> > -                       continue;
> > +                       if (mpol_equal(vma_policy(vma), new_pol))
> > +                               continue;
> > +                       /* vma_merge() joined vma && vma->next, case 8 */
>
> case 3 makes the same scenario?

Not really, afaics. "case 3" is when vma_merge() "merges" a hole with
vma, mbind_range() works with the already mmapped regions.

More precisely, unless I misread this code, "case 3" means area == next,
so vma_adjust(area) actually sets next->vm_start = addr.

I can be easily wrong, but to me vma_adjust() and its usage looks a bit
overcomplicated. Perhaps it makes sense to distinguish mmapped/hole cases.
mbind_range/madvise/etc need vma_join(vma, ...), not prev/anon_vma/file.
Perhaps. not sure.

> Acked-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>

Thanks!

Oleg.

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