linux-kernel - Re: [PATCH v2 1/3] pagemap: add mmap-exclusive bit for marking pages mapped only here

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Date:	Wed, 13 May 2015 13:59:41 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	linux-mm@...ck.org, Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mark Williamson <mwilliamson@...o-software.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	linux-api@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
	Vlastimil Babka <vbabka@...e.cz>, Pavel Machek <pavel@....cz>,
	Mark Seaborn <mseaborn@...omium.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Daniel James <djames@...o-software.com>,
	Finn Grimwood <fgrimwood@...o-software.com>
Subject: Re: [PATCH v2 1/3] pagemap: add mmap-exclusive bit for marking pages
 mapped only here

On 12.05.2015 13:40, Kirill A. Shutemov wrote:
> On Tue, May 12, 2015 at 12:43:03PM +0300, Konstantin Khlebnikov wrote:
>> This patch sets bit 56 in pagemap if this page is mapped only once.
>> It allows to detect exclusively used pages without exposing PFN:
>>
>> present file exclusive state
>> 0       0    0         non-present
>> 1       1    0         file page mapped somewhere else
>> 1       1    1         file page mapped only here
>> 1       0    0         anon non-CoWed page (shared with parent/child)
>> 1       0    1         anon CoWed page (or never forked)
>
> Probably, worth noting that file-private pages are anon in this context.
>

You mean there's another kind of CoW pages? Yep, but from the kernel
point of view these pages are the same. Anyway Userspace could look
into /proc/*/maps and see is there any file beyond anon vma.

-- 
Konstantin
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