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From:   Shakeel Butt <shakeelb@google.com>
Date:   Fri, 17 Jul 2020 08:17:09 -0700
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Subject: Re: Re: [PATCH v18 06/14] mm/damon: Implement callbacks for the
 virtual memory address spaces
To:     SeongJae Park <sjpark@amazon.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        SeongJae Park <sjpark@amazon.de>, Jonathan.Cameron@huawei.com,
        Andrea Arcangeli <aarcange@redhat.com>, acme@kernel.org,
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        Colin Ian King <colin.king@canonical.com>,
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On Thu, Jul 16, 2020 at 11:54 PM SeongJae Park <sjpark@amazon.com> wrote:
>
> On Thu, 16 Jul 2020 17:46:54 -0700 Shakeel Butt <shakeelb@google.com> wrote:
>
> > On Mon, Jul 13, 2020 at 1:44 AM SeongJae Park <sjpark@amazon.com> wrote:
> > >
> > > From: SeongJae Park <sjpark@amazon.de>
> > >
> > > This commit introduces a reference implementation of the address space
> > > specific low level primitives for the virtual address space, so that
> > > users of DAMON can easily monitor the data accesses on virtual address
> > > spaces of specific processes by simply configuring the implementation to
> > > be used by DAMON.
> > >
> > > The low level primitives for the fundamental access monitoring are
> > > defined in two parts:
> > > 1. Identification of the monitoring target address range for the address
> > > space.
> > > 2. Access check of specific address range in the target space.
> > >
> > > The reference implementation for the virtual address space provided by
> > > this commit is designed as below.
> > >
> > > PTE Accessed-bit Based Access Check
> > > -----------------------------------
> > >
> > > The implementation uses PTE Accessed-bit for basic access checks.  That
> > > is, it clears the bit for next sampling target page and checks whether
> > > it set again after one sampling period.  To avoid disturbing other
> > > Accessed bit users such as the reclamation logic, the implementation
> > > adjusts the ``PG_Idle`` and ``PG_Young`` appropriately, as same to the
> > > 'Idle Page Tracking'.
> > >
> > > VMA-based Target Address Range Construction
> > > -------------------------------------------
> > >
> > > Only small parts in the super-huge virtual address space of the
> > > processes are mapped to physical memory and accessed.  Thus, tracking
> > > the unmapped address regions is just wasteful.  However, because DAMON
> > > can deal with some level of noise using the adaptive regions adjustment
> > > mechanism, tracking every mapping is not strictly required but could
> > > even incur a high overhead in some cases.  That said, too huge unmapped
> > > areas inside the monitoring target should be removed to not take the
> > > time for the adaptive mechanism.
> > >
> > > For the reason, this implementation converts the complex mappings to
> > > three distinct regions that cover every mapped area of the address
> > > space.  Also, the two gaps between the three regions are the two biggest
> > > unmapped areas in the given address space.  The two biggest unmapped
> > > areas would be the gap between the heap and the uppermost mmap()-ed
> > > region, and the gap between the lowermost mmap()-ed region and the stack
> > > in most of the cases.  Because these gaps are exceptionally huge in
> > > usual address spacees, excluding these will be sufficient to make a
> > > reasonable trade-off.  Below shows this in detail::
> > >
> > >     <heap>
> > >     <BIG UNMAPPED REGION 1>
> > >     <uppermost mmap()-ed region>
> > >     (small mmap()-ed regions and munmap()-ed regions)
> > >     <lowermost mmap()-ed region>
> > >     <BIG UNMAPPED REGION 2>
> > >     <stack>
> > >
> > > Signed-off-by: SeongJae Park <sjpark@amazon.de>
> > > Reviewed-by: Leonard Foerster <foersleo@amazon.de>
> > [snip]
> > > +
> > > +static void damon_mkold(struct mm_struct *mm, unsigned long addr)
> > > +{
> > > +       pte_t *pte = NULL;
> > > +       pmd_t *pmd = NULL;
> > > +       spinlock_t *ptl;
> > > +
> > > +       if (follow_pte_pmd(mm, addr, NULL, &pte, &pmd, &ptl))
> > > +               return;
> > > +
> > > +       if (pte) {
> > > +               if (pte_young(*pte)) {
> >
> > Any reason for skipping mmu_notifier_clear_young()? Why exclude VMs as
> > DAMON's target applications?
>
> Obviously my mistake, thank you for pointing this!  I will add the function
> call in the next spin.
>

Similarly mmu_notifier_test_young() for the damon_young(). BTW I think
we can combine ctx->prepare_access_checks() and ctx->check_accesses()
into one i.e. get the young state for the previous cycle and mkold for
the next cycle in a single step.

I am wondering if there is any advantage to having "Page Idle
Tracking" beside DAMON. I think we can make them mutually exclusive.
Once we have established that I think DAMON can steal the two page
flag bits from it and can make use of them. What do you think?