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

[John Hubbard]

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.

thanks,
--
John Hubbard
NVIDIA