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From:   Shakeel Butt <shakeelb@google.com>
Date:   Wed, 25 Nov 2020 07:30:27 -0800
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Subject: Re: [PATCH v22 07/18] mm/page_idle: Avoid interferences from
 concurrent users
To:     SeongJae Park <sjpark@amazon.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
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On Tue, Oct 20, 2020 at 2:06 AM SeongJae Park <sjpark@amazon.com> wrote:
>
> From: SeongJae Park <sjpark@amazon.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@amazon.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?