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Date: Thu, 30 Apr 2015 14:43:56 +0300
Message-ID: <CALYGNiOcb2Z+zBVQ0Z058Z7+jp3EixYq8MK1f3MYYXOpT9iAiw@mail.gmail.com>
Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage
From: Konstantin Khlebnikov <koct9i@gmail.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Kirill A. Shutemov" <kirill@shutemov.name>,
        Mark Williamson <mwilliamson@undo-software.com>,
        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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On Thu, Apr 30, 2015 at 12:02 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Wed, Apr 29, 2015 at 1:44 PM, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
>>
>> This's no longer true. After recent fixes for "anon_vma endless growing" new vma
>> might reuse old anon_vma from grandparent vma.
>
> Oh well. I guess that was too simple.
>
> If Mark is ok with the rule that "it's not reliably if you have two
> nested forks" (ie it only works if you exec for every fork you do), it
> should still work, right? It sounds like Mark doesn't necessarily need
> to handle the *generic* case.

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.
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