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

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Message-ID: <CALCETrW7Ukh8KfKzpNgRc1D_5OK1o7bmEmFbtQTYoSoFiOSeKw@mail.gmail.com>
Date:	Fri, 26 Jul 2013 14:36:51 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Linux MM <linux-mm@...ck.org>, LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	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 Fri, Jul 26, 2013 at 2:18 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Fri, Jul 26, 2013 at 01:55:04PM -0700, 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
>
> It's the same as encoding pgoff in pte entry (pte is not present),
> but together with pgoff we save soft-bit status, later on #pf we decode
> pgoff and restore softbit back if it was there, pte itself can't disappear
> since it holds pgoff information.

Isn't that only the case for nonlinear mappings?

--Andy
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