linux-kernel - Re: [PATCH] mm: Save soft-dirty bits on swapped pages

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Message-ID: <CALCETrVPyHLid246Pw66cCENxM2L75Gm5kUR8fH549W_M1OAWA@mail.gmail.com>
Date:	Thu, 25 Jul 2013 09:02:32 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Pavel Emelyanov <xemul@...allels.com>
Cc:	Cyrill Gorcunov <gorcunov@...il.com>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Matt Mackall <mpm@...enic.com>,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...il.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>
Subject: Re: [PATCH] mm: Save soft-dirty bits on swapped pages

On Thu, Jul 25, 2013 at 12:29 AM, Pavel Emelyanov <xemul@...allels.com> wrote:
> On 07/24/2013 11:40 PM, Andy Lutomirski wrote:
>> On Wed, Jul 24, 2013 at 12:04 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>>> On Wed, Jul 24, 2013 at 10:55:41PM +0400, Pavel Emelyanov wrote:
>>>>>
>
>> 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.
>

Fair enough.  I'm still a little bit concerned that it will be hard
for userspace to distinguish between things for which soft-dirty works
(which will be more things once the patches are in) and things for
which soft-dirty doesn't work, assuming any are left.  But maybe this
is silly.

--Andy
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