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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Thu, 10 Jan 2019 23:08:07 -0800
Message-ID: <CAHk-=wh-kegfnPC_dmw0A72Sdk4B9tvce-cOR=jEfHDU1-4Eew@mail.gmail.com>
Subject: Re: [PATCH] mm/mincore: allow for making sys_mincore() privileged
To:     Dave Chinner <david@fromorbit.com>
Cc:     Dominique Martinet <asmadeus@codewreck.org>,
        Jiri Kosina <jikos@kernel.org>,
        Matthew Wilcox <willy@infradead.org>,
        Jann Horn <jannh@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Greg KH <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Michal Hocko <mhocko@suse.com>, Linux-MM <linux-mm@kvack.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>
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On Thu, Jan 10, 2019 at 8:04 PM Dave Chinner <david@fromorbit.com> wrote:
>
> So it will only read the single page we tried to access and won't
> perturb the rest of the message encoded into subsequent pages in
> file.

Dave, you're being intentionally obtuse, aren't you?

It's only that single page that *matters*. That's the page that the
probe reveals the status of - but it's also the page that the probe
then *changes* the status of.

See?

Think of it as "the act of measurement changes that which is
measured". And that makes the measurement pointless.

              Linus