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

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Date:	Thu, 25 Jul 2013 11:29:14 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Andy Lutomirski <luto@...capital.net>,
	Cyrill Gorcunov <gorcunov@...il.com>
CC:	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 07/24/2013 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
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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