Re: [PATCH 10/10] mm/hmm: add helpers for driver to safely take the mmap_sem

[Jerome Glisse <jglisse@redhat.com>]

On Wed, Feb 20, 2019 at 01:59:13PM -0800, John Hubbard wrote:
> On 1/29/19 8:54 AM, jglisse@redhat.com wrote:
> > From: Jérôme Glisse <jglisse@redhat.com>
> > 
> > The device driver context which holds reference to mirror and thus to
> > core hmm struct might outlive the mm against which it was created. To
> > avoid every driver to check for that case provide an helper that check
> > if mm is still alive and take the mmap_sem in read mode if so. If the
> > mm have been destroy (mmu_notifier release call back did happen) then
> > we return -EINVAL so that calling code knows that it is trying to do
> > something against a mm that is no longer valid.
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@redhat.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: Ralph Campbell <rcampbell@nvidia.com>
> > Cc: John Hubbard <jhubbard@nvidia.com>
> > ---
> >   include/linux/hmm.h | 50 ++++++++++++++++++++++++++++++++++++++++++---
> >   1 file changed, 47 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> > index b3850297352f..4a1454e3efba 100644
> > --- a/include/linux/hmm.h
> > +++ b/include/linux/hmm.h
> > @@ -438,6 +438,50 @@ struct hmm_mirror {
> >   int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm);
> >   void hmm_mirror_unregister(struct hmm_mirror *mirror);
> > +/*
> > + * hmm_mirror_mm_down_read() - lock the mmap_sem in read mode
> > + * @mirror: the HMM mm mirror for which we want to lock the mmap_sem
> > + * Returns: -EINVAL if the mm is dead, 0 otherwise (lock taken).
> > + *
> > + * The device driver context which holds reference to mirror and thus to core
> > + * hmm struct might outlive the mm against which it was created. To avoid every
> > + * driver to check for that case provide an helper that check if mm is still
> > + * alive and take the mmap_sem in read mode if so. If the mm have been destroy
> > + * (mmu_notifier release call back did happen) then we return -EINVAL so that
> > + * calling code knows that it is trying to do something against a mm that is
> > + * no longer valid.
> > + */
> 
> Hi Jerome,
> 
> Are you thinking that, throughout the HMM API, there is a problem that
> the mm may have gone away, and so driver code needs to be littered with
> checks to ensure that mm is non-NULL? If so, why doesn't HMM take a
> reference on mm->count?
> 
> This solution here cannot work. I think you'd need refcounting in order
> to avoid this kind of problem. Just doing a check will always be open to
> races (see below).
> 
> 
> > +static inline int hmm_mirror_mm_down_read(struct hmm_mirror *mirror)
> > +{
> > +	struct mm_struct *mm;
> > +
> > +	/* Sanity check ... */
> > +	if (!mirror || !mirror->hmm)
> > +		return -EINVAL;
> > +	/*
> > +	 * Before trying to take the mmap_sem make sure the mm is still
> > +	 * alive as device driver context might outlive the mm lifetime.
> > +	 *
> > +	 * FIXME: should we also check for mm that outlive its owning
> > +	 * task ?
> > +	 */
> > +	mm = READ_ONCE(mirror->hmm->mm);
> > +	if (mirror->hmm->dead || !mm)
> > +		return -EINVAL;
> > +
> 
> Nothing really prevents mirror->hmm->mm from changing to NULL right here.

This is really just to catch driver mistake, if driver does not call
hmm_mirror_unregister() then the !mm will never be true ie the
mirror->hmm->mm can not go NULL until the last reference to hmm_mirror
is gone.

> 
> > +	down_read(&mm->mmap_sem);
> > +	return 0;
> > +}
> > +
> 
> ...maybe better to just drop this patch from the series, until we see a
> pattern of uses in the calling code.

It use by nouveau now.

Cheers,
Jérôme