Re: [PATCH v2 hmm 09/11] mm/hmm: Poison hmm_range during unregister

[Jason Gunthorpe <jgg@ziepe.ca>]

On Thu, Jun 06, 2019 at 08:37:42PM -0700, John Hubbard wrote:
> On 6/6/19 11:44 AM, Jason Gunthorpe wrote:
> > From: Jason Gunthorpe <jgg@mellanox.com>
> > 
> > Trying to misuse a range outside its lifetime is a kernel bug. Use WARN_ON
> > and poison bytes to detect this condition.
> > 
> > Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
> > Reviewed-by: Jérôme Glisse <jglisse@redhat.com>
> > v2
> > - Keep range start/end valid after unregistration (Jerome)
> >  mm/hmm.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index 6802de7080d172..c2fecb3ecb11e1 100644
> > +++ b/mm/hmm.c
> > @@ -937,7 +937,7 @@ void hmm_range_unregister(struct hmm_range *range)
> >  	struct hmm *hmm = range->hmm;
> >  
> >  	/* Sanity check this really should not happen. */
> 
> That comment can also be deleted, as it has the same meaning as
> the WARN_ON() that you just added.
> 
> > -	if (hmm == NULL || range->end <= range->start)
> > +	if (WARN_ON(range->end <= range->start))
> >  		return;
> >  
> >  	mutex_lock(&hmm->lock);
> > @@ -948,7 +948,10 @@ void hmm_range_unregister(struct hmm_range *range)
> >  	range->valid = false;
> >  	mmput(hmm->mm);
> >  	hmm_put(hmm);
> > -	range->hmm = NULL;
> > +
> > +	/* The range is now invalid, leave it poisoned. */
> 
> To be precise, we are poisoning the range's back pointer to it's
> owning hmm instance.  Maybe this is clearer:
> 
> 	/*
> 	 * The range is now invalid, so poison it's hmm pointer. 
> 	 * Leave other range-> fields in place, for the caller's use.
> 	 */
> 
> ...or something like that?
> 
> > +	range->valid = false;
> > +	memset(&range->hmm, POISON_INUSE, sizeof(range->hmm));
> >  }
> >  EXPORT_SYMBOL(hmm_range_unregister);
> >  
> > 
> 
> The above are very minor documentation points, so:
> 
>     Reviewed-by: John Hubbard <jhubbard@nvidia.com>

done thanks

Jason