linux-kernel - Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap causes application-level breakage

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Date:	Thu, 30 Apr 2015 16:11:30 +0300
From:	Konstantin Khlebnikov <koct9i@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill@...temov.name>,
	Mark Williamson <mwilliamson@...o-software.com>,
	Mark Seaborn <mseaborn@...omium.org>,
	kernel list <linux-kernel@...r.kernel.org>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Linux API <linux-api@...r.kernel.org>,
	Finn Grimwood <fgrimwood@...o-software.com>,
	Daniel James <djames@...o-software.com>
Subject: Re: Regression: Requiring CAP_SYS_ADMIN for /proc/<pid>/pagemap
 causes application-level breakage

On Thu, Apr 30, 2015 at 2:43 PM, Konstantin Khlebnikov <koct9i@...il.com> wrote:
> On Thu, Apr 30, 2015 at 12:02 AM, Linus Torvalds
> <torvalds@...ux-foundation.org> wrote:
>> On Wed, Apr 29, 2015 at 1:44 PM, Konstantin Khlebnikov <koct9i@...il.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.

Something like this (see patch in attachment)

Download attachment "pagemap-add-mmap-shared-bit-for-marking-pages-mapped-multiple-times" of type "application/octet-stream" (3456 bytes)