Re: [PATCH 01/10] mm/hmm: use reference counting for HMM struct

[John Hubbard]

On 2/20/19 4:37 PM, Jerome Glisse wrote:
> On Wed, Feb 20, 2019 at 04:32:09PM -0800, John Hubbard wrote:
> > On 2/20/19 4:15 PM, Jerome Glisse wrote:
> > > On Wed, Feb 20, 2019 at 04:06:50PM -0800, John Hubbard wrote:
> > > > On 2/20/19 3:59 PM, Jerome Glisse wrote:
> > > > > On Wed, Feb 20, 2019 at 03:47:50PM -0800, John Hubbard wrote:
> > > > > > On 1/29/19 8:54 AM, jglisse@xxxxxxxxxx wrote:
> > > > > > > From: JÃrÃme Glisse <jglisse@xxxxxxxxxx>
> > > > > > >
> > > > > > > Every time i read the code to check that the HMM structure does not
> > > > > > > vanish before it should thanks to the many lock protecting its removal
> > > > > > > i get a headache. Switch to reference counting instead it is much
> > > > > > > easier to follow and harder to break. This also remove some code that
> > > > > > > is no longer needed with refcounting.
> > > > > >
> > > > > > Hi Jerome,
> > > > > >
> > > > > > That is an excellent idea. Some review comments below:
> > > > > >
> > > > > > [snip]
> > > > > >
> > > > > > >      static int hmm_invalidate_range_start(struct mmu_notifier *mn,
> > > > > > >      			const struct mmu_notifier_range *range)
> > > > > > >      {
> > > > > > >      	struct hmm_update update;
> > > > > > > -	struct hmm *hmm = range->mm->hmm;
> > > > > > > +	struct hmm *hmm = hmm_get(range->mm);
> > > > > > > +	int ret;
> > > > > > >      	VM_BUG_ON(!hmm);
> > > > > > > +	/* Check if hmm_mm_destroy() was call. */
> > > > > > > +	if (hmm->mm == NULL)
> > > > > > > +		return 0;
> > > > > >
> > > > > > Let's delete that NULL check. It can't provide true protection. If there
> > > > > > is a way for that to race, we need to take another look at refcounting.
> > > > >
> > > > > I will do a patch to delete the NULL check so that it is easier for
> > > > > Andrew. No need to respin.
> > > >
> > > > (Did you miss my request to make hmm_get/hmm_put symmetric, though?)
> > >
> > > Went over my mail i do not see anything about symmetric, what do you
> > > mean ?
> > >
> > > Cheers,
> > > JÃrÃme
> >
> > I meant the comment that I accidentally deleted, before sending the email!
> > doh. Sorry about that. :) Here is the recreated comment:
> >
> > diff --git a/mm/hmm.c b/mm/hmm.c
> > index a04e4b810610..b9f384ea15e9 100644
> >
> > --- a/mm/hmm.c
> >
> > +++ b/mm/hmm.c
> >
> > @@ -50,6 +50,7 @@
> >
> >   static const struct mmu_notifier_ops hmm_mmu_notifier_ops;
> >
> >    */
> >   struct hmm {
> >   	struct mm_struct	*mm;
> > +	struct kref		kref;
> >   	spinlock_t		lock;
> >   	struct list_head	ranges;
> >   	struct list_head	mirrors;
> >
> > @@ -57,6 +58,16 @@
> >
> >   struct hmm {
> >
> >   	struct rw_semaphore	mirrors_sem;
> >   };
> >
> > +static inline struct hmm *hmm_get(struct mm_struct *mm)
> > +{
> > +	struct hmm *hmm = READ_ONCE(mm->hmm);
> > +
> > +	if (hmm && kref_get_unless_zero(&hmm->kref))
> > +		return hmm;
> > +
> > +	return NULL;
> > +}
> > +
> >
> > So for this, hmm_get() really ought to be symmetric with
> > hmm_put(), by taking a struct hmm*. And the null check is
> > not helping here, so let's just go with this smaller version:
> >
> > static inline struct hmm *hmm_get(struct hmm *hmm)
> > {
> > 	if (kref_get_unless_zero(&hmm->kref))
> > 		return hmm;
> >
> > 	return NULL;
> > }
> >
> > ...and change the few callers accordingly.
>
> What about renaning hmm_get() to mm_get_hmm() instead ?

For a get/put pair of functions, it would be ideal to pass
the same argument type to each. It looks like we are passing
around hmm*, and hmm retains a reference count on hmm->mm,
so I think you have a choice of using either mm* or hmm* as
the argument. I'm not sure that one is better than the other
here, as the lifetimes appear to be linked pretty tightly.

Whichever one is used, I think it would be best to use it
in both the _get() and _put() calls.

thanks,
--
John Hubbard
NVIDIA