Re: [PATCH v23 05/15] mm/damon: Implement primitives for the virtual memory address spaces

[Shakeel Butt <shakeelb@google.com>]

On Wed, Dec 23, 2020 at 8:47 AM SeongJae Park <sjpark@amazon.com> wrote:
>
[snip]
> > [snip]
> > > +
> > > +static bool damon_va_young(struct mm_struct *mm, unsigned long addr,
> > > +                       unsigned long *page_sz)
> > > +{
> > > +       pte_t *pte = NULL;
> > > +       pmd_t *pmd = NULL;
> > > +       spinlock_t *ptl;
> > > +       bool young = false;
> > > +
> > > +       if (follow_pte_pmd(mm, addr, NULL, &pte, &pmd, &ptl))
> > > +               return false;
> > > +
> > > +       *page_sz = PAGE_SIZE;
> > > +       if (pte) {
> > > +               young = pte_young(*pte);
> > > +               if (!young)
> > > +                       young = !page_is_idle(pte_page(*pte));
> > > +               pte_unmap_unlock(pte, ptl);
> > > +               return young;
> > > +       }
> > > +
> > > +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
> > > +       young = pmd_young(*pmd);
> > > +       if (!young)
> > > +               young = !page_is_idle(pmd_page(*pmd));
> > > +       spin_unlock(ptl);
> > > +       *page_sz = ((1UL) << HPAGE_PMD_SHIFT);
> > > +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
> > > +
> > > +       return young;
> >
> > You need mmu_notifier_test_young() here. Hmm I remember mentioning
> > this in some previous version as well.
>
> Your question and my answer was as below:
>
>     > Don't you need mmu_notifier_clear_young() here?
>
>     I think we don't need it here because we only read the Accessed bit and PG_Idle
>     if Accessed bit was not set.
>
> I should notice that you mean 'test_young()' but didn't, sorry.  I will add it
> in the next version.
>

I should have said mmu_notifier_test_young() instead of
mmu_notifier_clear_young().

> >
> > BTW have you tested this on a VM?
>
> Yes.  Indeed, I'm testing this on a QEMU/KVM environment.  You can get more
> detail at: https://damonitor.github.io/doc/html/latest/vm/damon/eval.html#setup
>

Hmm without mmu_notifier_test_young() you should be missing the kvm
mmu access updates. Can you please recheck if your eval is correctly
seeing the memory accesses from the VM?