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

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Message-ID: <51F0232D.6060306@parallels.com>
Date:	Wed, 24 Jul 2013 22:55:41 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	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 10:52 PM, Cyrill Gorcunov wrote:
> On Wed, Jul 24, 2013 at 11:21:46AM -0700, Andy Lutomirski wrote:
>>>
>>> I fear for tracking soft-dirty-bit for swapped entries we sinply have
>>> no other place than pte (still i'm quite open for ideas, maybe there
>>> are a better way which I've missed).
>>
>> I know approximately nothing about how swap and anon_vma work.
>>
>> For files, sticking it in struct page seems potentially nicer,
>> although finding a free bit might be tough.  (FWIW, I have plans to
>> free up a page flag on x86 some time moderately soon as part of a
>> completely unrelated project.)  I think this stuff really belongs to
>> the address_space more than it belongs to the pte.
> 
> 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?

The soft-dirty thing watches changes in the virtual memory, not in
the physical one.

>>
>> How do you handle the write syscall?
> 
> I fear I somehow miss your point here, could please alaborate a bit?
> There is no additional code I know of being write() specific, just
> a code for #PF exceptions.
> .
> 


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