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    SubjectRe: [PATCH 01/10] mm/hmm: use reference counting for HMM struct
    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@redhat.com wrote:
    > > > > > > From: Jérôme Glisse <jglisse@redhat.com>
    > > > > > >
    > > > > > > 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

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