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From: Mark Seaborn <mseaborn@chromium.org>
Date: Fri, 24 Apr 2015 08:26:52 -0700
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Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage
To: Mark Williamson <mwilliamson@undo-software.com>
Cc: kernel list <linux-kernel@vger.kernel.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Pavel Emelyanov <xemul@parallels.com>,
        Konstantin Khlebnikov <khlebnikov@openvz.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@amacapital.net>, linux-api@vger.kernel.org,
        Finn Grimwood <fgrimwood@undo-software.com>,
        Daniel James <djames@undo-software.com>
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On 24 April 2015 at 08:01, Mark Williamson
<mwilliamson@undo-software.com> wrote:
> In our use of /proc/PID/pagemap, we currently make use of the physical
> pageframe addresses.  We should be able to work with a scrambled
> representation of these (Andy Lutomirski suggested this in the
> original discussion - https://lkml.org/lkml/2015/3/16/1273) so long as
> the scrambling remained consistent during the lifetime of the open
> pagemap file.  Alternatively, if physical addresses were simply zeroed
> (also suggested by Pavel Emelyanov -
> https://lkml.org/lkml/2015/3/9/871) we would be able to change our
> code to rely only on the soft-dirty flag and thus still work
> correctly.

I'm curious, what do you use the physical page addresses for?

Since you pointed to http://undo-software.com, which talks about
reversible debugging tools, I can guess you would use the soft-dirty
flag to implement copy-on-write snapshotting.  I'm guessing you might
use physical page addresses for determining when the same page is
mapped twice (in the same process or different processes)?

Cheers,
Mark
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