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, [email protected] wrote: > > > > > > From: Jérôme Glisse <[email protected]> > > > > > > > > > > > > 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
