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

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