From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1754699Ab3GYH3n (ORCPT <rfc822;w@1wt.eu>);
	Thu, 25 Jul 2013 03:29:43 -0400
Received: from relay.parallels.com ([195.214.232.42]:54807 "EHLO
	relay.parallels.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1753096Ab3GYH3l (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Thu, 25 Jul 2013 03:29:41 -0400
Message-ID: <51F0D3CA.3080902@parallels.com>
Date: Thu, 25 Jul 2013 11:29:14 +0400
From: Pavel Emelyanov <xemul@parallels.com>
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0
MIME-Version: 1.0
To: Andy Lutomirski <luto@amacapital.net>,
        Cyrill Gorcunov <gorcunov@gmail.com>
CC: James Bottomley <James.Bottomley@hansenpartnership.com>,
        Linux MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matt Mackall <mpm@selenic.com>,
        Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>,
        Marcelo Tosatti <mtosatti@redhat.com>,
        KOSAKI Motohiro <kosaki.motohiro@gmail.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>
Subject: Re: [PATCH] mm: Save soft-dirty bits on swapped pages
References: <CALCETrXYnkonpBANnUuX+aJ=B=EYFwecZO27yrqcEU8WErz9DA@mail.gmail.com> <20130724163734.GE24851@moon> <CALCETrVWgSMrM2ujpO092ZLQa3pWEQM4vdmHhCVUohUUcoR8AQ@mail.gmail.com> <20130724171728.GH8508@moon> <1374687373.7382.22.camel@dabdike> <CALCETrV5MD1qCQsyz4=t+QW1BJuTBYainewzDfEaXW12S91K=A@mail.gmail.com> <20130724181516.GI8508@moon> <CALCETrV5NojErxWOc2RpuYKE0g8FfOmKB31oDz46CRu27hmDBA@mail.gmail.com> <20130724185256.GA24365@moon> <51F0232D.6060306@parallels.com> <20130724190453.GJ8508@moon> <CALCETrVRQBLrQBL8_Zu0VqBRkDXXr2np57-gt4T59A4jG9jMZw@mail.gmail.com>
In-Reply-To: <CALCETrVRQBLrQBL8_Zu0VqBRkDXXr2np57-gt4T59A4jG9jMZw@mail.gmail.com>
Content-Type: text/plain; charset="ISO-8859-1"
Content-Transfer-Encoding: 7bit
X-Originating-IP: [89.169.95.100]
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On 07/24/2013 11:40 PM, Andy Lutomirski wrote:
> On Wed, Jul 24, 2013 at 12:04 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
>> On Wed, Jul 24, 2013 at 10:55:41PM +0400, Pavel Emelyanov wrote:
>>>>
>>>> Well, some part of information already lays in pte (such as 'file' bit,
>>>> swap entries) so it looks natural i think to work on this level. but
>>>> letme think if use page struct for that be more convenient...
>>>
>>> It hardly will be. Consider we have a page shared between two tasks,
>>> then first one "touches" it and soft-dirty is put onto his PTE and,
>>> subsequently, the page itself. The we go and clear sofr-dirty for the
>>> 2nd task. What should we do with the soft-dirty bit on the page?
>>
>> Indeed, this won't help. Well then, bippidy-boppidy-boo, our
>> pants are metaphorically on fire (c)
> 
> Hmm.  So there are at least three kinds of memory:
> 
> Anonymous pages: soft-dirty works
> Shared file-backed pages: soft-dirty does not work
> Private file-backed pages: soft-dirty works (but see below)

The shared file-backed pages case works, but unmap-map case doesn't
preserve the soft-dirty bit. Just like the private file did. We'll
fix this case next.

> Perhaps another bit should be allocated to expose to userspace either
> "soft-dirty", "soft-clean", or "soft-dirty unsupported"?
> 
> There's another possible issue with private file-backed pages, though:
> how do you distinguish clean-and-not-cowed from cowed-but-soft-clean?
> (The former will reflect changes in the underlying file, I think, but
> the latter won't.)

There's a bit called PAGE_FILE bit in /proc/pagemap file introduced with
the 052fb0d635df5d49dfc85687d94e1a87bf09378d commit.

Plz, refer to Documentation/vm/pagemap.txt and soft-dirty.txt, all this
is described there pretty well.

> --Andy

Thanks,
Pavel