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

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Date:	Thu, 25 Jul 2013 16:26:44 +0800
From:	Hush Bensen <hush.bensen@...il.com>
To:	Pavel Emelyanov <xemul@...allels.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	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 07/25/2013 03:29 PM, Pavel Emelyanov 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:
>>>>> 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

What's the meaning of unmap-map case?

> 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
>
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