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From:   SeongJae Park <sjpark@amazon.com>
To:     Shakeel Butt <shakeelb@google.com>
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        Andrew Morton <akpm@linux-foundation.org>,
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Subject: Re: Re: [PATCH v18 06/14] mm/damon: Implement callbacks for the virtual memory address spaces
Date:   Fri, 17 Jul 2020 08:53:13 +0200
Message-ID: <20200717065313.8870-1-sjpark@amazon.com>
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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.


Thanks,
SeongJae Park

> 
> > +                       clear_page_idle(pte_page(*pte));
> > +                       set_page_young(pte_page(*pte));
> > +               }
> > +               *pte = pte_mkold(*pte);
> > +               pte_unmap_unlock(pte, ptl);
> > +               return;
> > +       }
> > +