linux-kernel - Re: [PATCH v22 07/18] mm/page_idle: Avoid interferences from concurrent users

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Date:   Wed, 25 Nov 2020 07:30:27 -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 v22 07/18] mm/page_idle: Avoid interferences from
 concurrent users

On Tue, Oct 20, 2020 at 2:06 AM SeongJae Park <sjpark@...zon.com> wrote:
>
> From: SeongJae Park <sjpark@...zon.de>
>
> Concurrent Idle Page Tracking users can interfere each other because the
> interface doesn't provide a central rule for synchronization between the
> users.  Users could implement their own synchronization rule, but even
> in that case, applications developed by different users would not know
> how to synchronize with others.  To help this situation, this commit
> introduces a centralized synchronization infrastructure of Idle Page
> Tracking.
>
> In detail, this commit introduces a mutex lock for Idle Page Tracking,
> called 'page_idle_lock'.  It is exposed to user space via a new bool
> sysfs file, '/sys/kernel/mm/page_idle/lock'.  By writing to and reading
> from the file, users can hold/release and read status of the mutex.
> Writes to the Idle Page Tracking 'bitmap' file fails if the lock is not
> held, while reads of the file can be done regardless of the lock status.
>
> Note that users could still interfere each other if they abuse this
> locking rule.  Nevertheless, this change will let them notice the rule.
>
> Signed-off-by: SeongJae Park <sjpark@...zon.de>

I don't think this is allowed. I mean returning to user space with
held mutex and other processes can unlock it. I don't think mutex is
what you need. Or more importantly is this really an issue?