linux-kernel - Re: [PATCH v31 05/13] mm/damon: Implement primitives for the virtual memory address spaces

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Date:   Wed, 30 Jun 2021 17:19:06 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     SeongJae Park <sj38.park@...il.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        SeongJae Park <sjpark@...zon.de>, Jonathan.Cameron@...wei.com,
        acme@...nel.org, alexander.shishkin@...ux.intel.com,
        amit@...nel.org, benh@...nel.crashing.org,
        Brendan Higgins <brendanhiggins@...gle.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@...ah.com,
        Greg Thelen <gthelen@...gle.com>, guoju.fgj@...baba-inc.com,
        jgowans@...zon.com, Mel Gorman <mgorman@...e.de>, mheyne@...zon.de,
        Minchan Kim <minchan@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, namhyung@...nel.org,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Rik van Riel <riel@...riel.com>,
        David Rientjes <rientjes@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mike Rapoport <rppt@...nel.org>, Shuah Khan <shuah@...nel.org>,
        sieberf@...zon.com, snu@...le79.org,
        Vlastimil Babka <vbabka@...e.cz>,
        Vladimir Davydov <vdavydov.dev@...il.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 v31 05/13] mm/damon: Implement primitives for the virtual
 memory address spaces

On Wed, Jun 30, 2021 at 5:18 PM Shakeel Butt <shakeelb@...gle.com> wrote:
>
> On Mon, Jun 21, 2021 at 1:31 AM SeongJae Park <sj38.park@...il.com> wrote:
> >
> > From: SeongJae Park <sjpark@...zon.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@...zon.de>
> > Reviewed-by: Leonard Foerster <foersleo@...zon.de>
> > Reviewed-by: Fernand Sieber <sieberf@...zon.com>
>
> Acked-by: Shakeel Butt <shakeelb@...gle.com>

Ok that was by mistake. The ACK is for v32.