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

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Message-ID: <51F41FA0.6060205@parallels.com>
Date:	Sat, 27 Jul 2013 23:29:36 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	Cyrill Gorcunov <gorcunov@...il.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 file pages

On 07/27/2013 12:55 AM, Andy Lutomirski wrote:
> On Fri, Jul 26, 2013 at 1:18 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>> Andy reported that if file page get reclaimed we loose soft-dirty bit
>> if it was there, so save _PAGE_BIT_SOFT_DIRTY bit when page address
>> get encoded into pte entry. Thus when #pf happens on such non-present
>> pte we can restore it back.
>>
> 
> Unless I'm misunderstanding this, it's saving the bit in the
> non-present PTE.  This sounds wrong -- what happens if the entire pmd
> (or whatever the next level is called) gets zapped?  (Also, what
> happens if you unmap a file and map a different file there?)

The whole pte gets zapped on vma unmap, and in this case forgetting
the soft-dirty bit completely is OK.

> --Andy
> .
> 


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