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: Jérôme 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,
> > Jérôme
>
> 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 ?
Cheers,
Jérôme
