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Date:   Wed, 20 Feb 2019 18:09:54 -0500
From:   Jerome Glisse <jglisse@...hat.com>
To:     John Hubbard <jhubbard@...dia.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ralph Campbell <rcampbell@...dia.com>
Subject: Re: [PATCH 10/10] mm/hmm: add helpers for driver to safely take the
 mmap_sem

On Wed, Feb 20, 2019 at 02:40:20PM -0800, John Hubbard wrote:
> On 2/20/19 2:19 PM, Jerome Glisse wrote:
> > On Wed, Feb 20, 2019 at 01:59:13PM -0800, John Hubbard wrote:
> > > On 1/29/19 8:54 AM, jglisse@...hat.com wrote:
> > > > From: Jérôme Glisse <jglisse@...hat.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 something against a mm that is no longer valid.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@...hat.com>
> > > > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > > > Cc: Ralph Campbell <rcampbell@...dia.com>
> > > > Cc: John Hubbard <jhubbard@...dia.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.
> 
> In that case, then this again seems unnecessary, and in fact undesirable.
> If the driver code has a bug, then let's let the backtrace from a NULL
> dereference just happen, loud and clear.
> 
> This patch, at best, hides bugs. And it adds code that should simply be
> unnecessary, so I don't like it. :)  Let's make it go away.
> 
> > 
> > > 
> > > > +	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.
> 
> Maybe you'd have to remove that use case in a couple steps, depending on the
> order that patches are going in.

Well all that is needed is removing if (mirror->hmm->dead || !mm) return -EINVAL;
from functions so it does not have any ordering conflict with anything really.

Cheers,
Jérôme

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