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From:   SeongJae Park <sjpark@amazon.com>
To:     SeongJae Park <sjpark@amazon.com>
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Subject: Re: [PATCH v23 05/15] mm/damon: Implement primitives for the virtual memory address spaces
Date:   Wed, 27 Jan 2021 17:56:30 +0100
Message-ID: <20210127165630.29904-1-sjpark@amazon.com>
In-Reply-To: <20201224071111.11551-1-sjpark@amazon.com>
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Content-Type: text/plain
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On Thu, 24 Dec 2020 08:11:11 +0100 SeongJae Park <sjpark@amazon.com> wrote:

> On Wed, 23 Dec 2020 14:54:02 -0800 Shakeel Butt <shakeelb@google.com> wrote:
> 
> > 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?
> 
> Seems I didn't clearly answered, sorry.  My test setup installs the
> DAMON-enabled kernel in a guest VM and run it for workloads in the guest,
> rather than running DAMON in host to monitor accesses of VMs.  The MMU notifier
> is for latter case, AFAIU, so my test setup didn't see the problem.

Just FYI.  I confirmed the mmu_notifier_test_young() added version works for
the use case.  I tested it by running a program accessing 200MB memory in a
QEMU/KVM guest having 120GB memory and monitoring the qemu process' virtual
address space from the host using DAMON.  The 200MB memory region was clearly
identifiable.

Thanks,
SeongJae Park