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

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Message-ID: <5555BEB9.6000905@yandex-team.ru>
Date:	Fri, 15 May 2015 12:39:05 +0300
From:	Konstantin Khlebnikov <khlebnikov@...dex-team.ru>
To:	Mark Williamson <mwilliamson@...o-software.com>
CC:	linux-mm@...ck.org, Naoya Horiguchi <n-horiguchi@...jp.nec.com>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	Linux API <linux-api@...r.kernel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Vlastimil Babka <vbabka@...e.cz>, Pavel Machek <pavel@....cz>,
	Mark Seaborn <mseaborn@...omium.org>,
	"Kirill A. Shutemov" <kirill@...temov.name>,
	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 14.05.2015 21:50, Mark Williamson wrote:
> Hi Konstantin,
>
> On Wed, May 13, 2015 at 11:51 AM, Konstantin Khlebnikov
> <khlebnikov@...dex-team.ru> wrote:
>> On 12.05.2015 15:05, Mark Williamson wrote:
> <snip>
>>>    1. I was hoping we'd be able to backport a compatible fix to older
>>> kernels that might adopt the pagemap permissions change.  Using the V2
>>> format flags rules out doing this for kernels that are too old to have
>>> soft-dirty, I think.
>>>
>>>    2. From our software's PoV, I feel it's worth noting that it doesn't
>>> strictly fix ABI compatibility, though I realise that's probably not
>>> your primary concern here.  We'll need to modify our code to write the
>>> clear_refs file but that change is OK for us if it's the preferred
>>> solution.
> <snip>
>> I prefer to backport v2 format (except soft-dirty bit and clear_refs)
>> into older kernels. Page-shift bits are barely used so nobody will see
>> the difference.
>
> My concern was whether a change to format would be acceptable to
> include in the various -stable kernels; they are already including the
> additional protections on pagemap, so we're starting to need our
> fallback mode in distributions.  Do you think that such a patch would
> be acceptable there?
>
> (As an application vendor we're likely to be particularly stuck with
> what the commercial distributions decide to ship, which is why I'm
> trying to keep an eye on this)
>
> I appreciate that this is a slightly administrative concern!  I
> definitely like the technical approach of this code and it seems to
> work fine for us.

I cannot guarantee that v2 format will be accepted into stable kernels
and into distributives. I'm not the gate keeper.

As a fallback probably you should invent some kind of suid helper
which gives you access to required information without exposing pfn.
For example: it gets pids and memory ranges as arguments and prints
bitmap of CoWed pages into stdout.

-- 
Konstantin
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