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

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Date:   Wed, 27 Jan 2021 09:02:11 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     SeongJae Park <sjpark@...zon.com>
Cc:     Jonathan.Cameron@...wei.com,
        Andrea Arcangeli <aarcange@...hat.com>, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, amit@...nel.org,
        benh@...nel.crashing.org, brendan.d.gregg@...il.com,
        Brendan Higgins <brendanhiggins@...gle.com>,
        Qian Cai <cai@....pw>,
        Colin Ian King <colin.king@...onical.com>,
        Jonathan Corbet <corbet@....net>,
        David Hildenbrand <david@...hat.com>, dwmw@...zon.com,
        Marco Elver <elver@...gle.com>, "Du, Fan" <fan.du@...el.com>,
        foersleo@...zon.de, Greg Thelen <gthelen@...gle.com>,
        Ian Rogers <irogers@...gle.com>, jolsa@...hat.com,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Mark Rutland <mark.rutland@....com>,
        Mel Gorman <mgorman@...e.de>, Minchan Kim <minchan@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, namhyung@...nel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        David Rientjes <rientjes@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mike Rapoport <rppt@...nel.org>, sblbir@...zon.com,
        Shuah Khan <shuah@...nel.org>, sj38.park@...il.com,
        snu@...zon.de, Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        Huang Ying <ying.huang@...el.com>, zgf574564920@...il.com,
        linux-damon@...zon.com, Linux MM <linux-mm@...ck.org>,
        linux-doc@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v23 05/15] mm/damon: Implement primitives for the virtual
 memory address spaces

On Wed, Jan 27, 2021 at 8:57 AM SeongJae Park <sjpark@...zon.com> wrote:
>
> On Thu, 24 Dec 2020 08:11:11 +0100 SeongJae Park <sjpark@...zon.com> wrote:
>
> > On Wed, 23 Dec 2020 14:54:02 -0800 Shakeel Butt <shakeelb@...gle.com> wrote:
> >
> > > On Wed, Dec 23, 2020 at 8:47 AM SeongJae Park <sjpark@...zon.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 for confirming. I am still going over the whole series and will
send out the emails in a couple of days.

Shakeel