linux-kernel - Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with existing use

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Message-ID: <20130822054919.GF18673@moon>
Date:	Thu, 22 Aug 2013 09:49:19 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Andy Lutomirski <luto@...capital.net>,
	Pavel Emelyanov <xemul@...allels.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...hat.com>, Xen-devel@...ts.xen.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Jan Beulich <jbeulich@...e.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with
 existing use

On Wed, Aug 21, 2013 at 04:42:53PM -0700, Andi Kleen wrote:
> Cyrill Gorcunov <gorcunov@...il.com> writes:
> >
> > Hi all, I worked on patch which would not touch PSE bit for dirty page
> > tracking and the result is not that good:
> >
> >  - 2level pages now always page dirty if page is swapped in and out, because
> >    there is no space left in PTE (other than PSE bit)
> 
> Maybe just don't support soft dirty for 2 level page tables?
> 
> 2 level page tables should be really on the way out anyways, as they
> have severe limits and do not support NX. With 3 levels there is enough
> space.

Look, good thing is that 7th bit also available on the 4level pages
(ie x86-64) without additional code modification, that's why I picked
it in first place. I prepared the patch locally which doesn't use
pse bit for tracking but it only makes code more complex.
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