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Date: Thu, 30 Apr 2015 19:32:29 +0100
Message-ID: <CAEVpBaJrXz5Xc5oH4-DcerC=Y0o8r9cbjVgVSqVJDuN1453jUg@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: Konstantin Khlebnikov <koct9i@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill@shutemov.name>,
        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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Hi all,

On Thu, Apr 30, 2015 at 2:11 PM, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
> On Thu, Apr 30, 2015 at 2:43 PM, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
>> What about exposing shared/exclusive bit in pagemap == 1 if
>> page_mapcount() > 1, otherwise 0 (or vise versa).
>>
>> Seems like this should work for detecting CoWed pages in child mm.
>
> Something like this (see patch in attachment)

Either something like this patch (updated to cover THPs), or Linus's
suggestion seems worth a try.  Could I perhaps get a steer on which
would be more likely to be accepted / preferred?

Either way, we'd want to expose the resulting flag somewhere within
pagemap.  We could do this either within the normal flags region, or
potentially even repurpose one of the (now censored) physical bits.

If there's a general feeling then I'll update my work-in-progress and
post it here.

Thanks,
Mark
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