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Date: Thu, 30 Apr 2015 19:45:06 +0100
Message-ID: <CAEVpBaLQx-8Hxjav5OO0xntDs=2wEXJwLXXW3FuNE0yCeBjBeQ@mail.gmail.com>
Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage
From: Mark Williamson <mwilliamson@undo-software.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Konstantin Khlebnikov <koct9i@gmail.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Mark Seaborn <mseaborn@chromium.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Pavel Emelyanov <xemul@parallels.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>,
        Linux API <linux-api@vger.kernel.org>,
        Finn Grimwood <fgrimwood@undo-software.com>,
        Daniel James <djames@undo-software.com>
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>> Something like this (see patch in attachment)
>
> THP is not covered.
>
> Any comments on kcmp() idea?

It seems like a modified kcmp() would also be a valid approach but, as
you noted, probably speed-limited for our purposes.  As you say, there
is the option of a vector-oriented equivalent.  It seems like a
generally nice facility to have available in the kernel but my
suspicion is that it wouldn't be optimal for us...

My thinking is that using soft-dirty might give us the best
performance on platforms where it's available.  We're only using
fork() as a cunning/hacky way of tracking what pages change;
soft-dirty would allow us to avoid that too, whereas using kcmp()
would still require the forking overhead.

Does that make sense, or have I missed something?

Thanks,
Mark
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