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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 24 Jul 2013 12:40:22 -0700
Message-ID: <CALCETrVRQBLrQBL8_Zu0VqBRkDXXr2np57-gt4T59A4jG9jMZw@mail.gmail.com>
Subject: Re: [PATCH] mm: Save soft-dirty bits on swapped pages
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To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: Pavel Emelyanov <xemul@parallels.com>, 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>

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)

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.)

--Andy

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