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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 30 Jun 2021 17:19:38 -0700
Message-ID: <CALvZod4zm0KjGMZzooOPXXDJPyw-TaFSeJn-s-tL3EWBz2td_A@mail.gmail.com>
Subject: Re: [PATCH v32 05/13] mm/damon: Implement primitives for the virtual
 memory address spaces
To:     SeongJae Park <sj38.park@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        SeongJae Park <sjpark@amazon.de>, Jonathan.Cameron@huawei.com,
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        Jonathan Corbet <corbet@lwn.net>,
        David Hildenbrand <david@redhat.com>, dwmw@amazon.com,
        Marco Elver <elver@google.com>, "Du, Fan" <fan.du@intel.com>,
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        Greg Thelen <gthelen@google.com>, guoju.fgj@alibaba-inc.com,
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        Ingo Molnar <mingo@redhat.com>, namhyung@kernel.org,
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        Vlastimil Babka <vbabka@suse.cz>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
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On Mon, Jun 28, 2021 at 6:34 AM SeongJae Park <sj38.park@gmail.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 does the
> works 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 the next sampling target page and checks
> whether it is set again after one sampling period.  This could disturb
> the reclaim logic.  DAMON uses ``PG_idle`` and ``PG_young`` page flags
> to solve the conflict, as Idle page tracking does.
>
> 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 spaces, 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>
> Reviewed-by: Fernand Sieber <sieberf@amazon.com>

Acked-by: Shakeel Butt <shakeelb@google.com>