Re: [PATCH 04/15] mm/hmm: unregister mmu_notifier when last HMM client quit v2

[Jerome Glisse]

On Wed, Mar 21, 2018 at 04:22:49PM -0700, John Hubbard wrote:
> On 03/21/2018 11:16 AM, jglisse@xxxxxxxxxx wrote:
> > From: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > 
> > This code was lost in translation at one point. This properly call
> > mmu_notifier_unregister_no_release() once last user is gone. This
> > fix the zombie mm_struct as without this patch we do not drop the
> > refcount we have on it.
> > 
> > Changed since v1:
> >   - close race window between a last mirror unregistering and a new
> >     mirror registering, which could have lead to use after free()
> >     kind of bug
> > 
> > Signed-off-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
> > Cc: Evgeny Baskakov <ebaskakov@xxxxxxxxxx>
> > Cc: Ralph Campbell <rcampbell@xxxxxxxxxx>
> > Cc: Mark Hairgrove <mhairgrove@xxxxxxxxxx>
> > Cc: John Hubbard <jhubbard@xxxxxxxxxx>
> > ---
> >  mm/hmm.c | 35 +++++++++++++++++++++++++++++++++--
> >  1 file changed, 33 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 6088fa6ed137..f75aa8df6e97 100644
> > --- a/mm/hmm.c
> > +++ b/mm/hmm.c
> > @@ -222,13 +222,24 @@ int hmm_mirror_register(struct hmm_mirror *mirror, struct mm_struct *mm)
> >  	if (!mm || !mirror || !mirror->ops)
> >  		return -EINVAL;
> >  
> > +again:
> >  	mirror->hmm = hmm_register(mm);
> >  	if (!mirror->hmm)
> >  		return -ENOMEM;
> >  
> >  	down_write(&mirror->hmm->mirrors_sem);
> > -	list_add(&mirror->list, &mirror->hmm->mirrors);
> > -	up_write(&mirror->hmm->mirrors_sem);
> > +	if (mirror->hmm->mm == NULL) {
> > +		/*
> > +		 * A racing hmm_mirror_unregister() is about to destroy the hmm
> > +		 * struct. Try again to allocate a new one.
> > +		 */
> > +		up_write(&mirror->hmm->mirrors_sem);
> > +		mirror->hmm = NULL;
> 
> This is being set outside of locks, so now there is another race with
> another hmm_mirror_register...
> 
> I'll take a moment and draft up what I have in mind here, which is a more
> symmetrical locking scheme for these routines.
> 

No this code is correct. hmm->mm is set after hmm struct is allocated
and before it is public so no one can race with that. It is clear in
hmm_mirror_unregister() under the write lock hence checking it here
under that same lock is correct.

Cheers,
Jérôme