linux-kernel - Re: [PATCH v23 04/15] mm/idle_page_tracking: Make PG

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Date:   Wed, 23 Dec 2020 07:11:19 -0800
From:   Shakeel Butt <shakeelb@...gle.com>
To:     SeongJae Park <sjpark@...zon.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        SeongJae Park <sjpark@...zon.de>, 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 04/15] mm/idle_page_tracking: Make PG_idle reusable

On Tue, Dec 15, 2020 at 3:57 AM SeongJae Park <sjpark@...zon.com> wrote:
>
> From: SeongJae Park <sjpark@...zon.de>
>
> PG_idle and PG_young allow the two PTE Accessed bit users, Idle Page
> Tracking and the reclaim logic concurrently work while don't interfere
> each other.  That is, when they need to clear the Accessed bit, they set
> PG_young to represent the previous state of the bit, respectively.  And
> when they need to read the bit, if the bit is cleared, they further read
> the PG_young to know whether the other has cleared the bit meanwhile or
> not.
>
> We could add another page flag and extend the mechanism to use the flag
> if we need to add another concurrent PTE Accessed bit user subsystem.
> However, the space is limited.  Meanwhile, if the new subsystem is
> mutually exclusive with IDLE_PAGE_TRACKING or interfering with it is not
> a real problem, it would be ok to simply reuse the PG_idle flag.
> However, it's impossible because the flags are dependent on
> IDLE_PAGE_TRACKING.
>
> To allow such reuse of the flags, this commit separates the PG_young and
> PG_idle flag logic from IDLE_PAGE_TRACKING and introduces new kernel
> config, 'PAGE_IDLE_FLAG'.  Hence, a new subsystem would be able to reuse
> PG_idle without depending on IDLE_PAGE_TRACKING.
>
> In the next commit, DAMON's reference implementation of the virtual
> memory address space monitoring primitives will use it.
>
> Signed-off-by: SeongJae Park <sjpark@...zon.de>

Reviewed-by: Shakeel Butt <shakeelb@...gle.com>