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

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Date:	Thu, 22 Aug 2013 10:41:38 +0400
From:	Pavel Emelyanov <xemul@...allels.com>
To:	Dave Jones <davej@...hat.com>, Cyrill Gorcunov <gorcunov@...il.com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Jan Beulich <JBeulich@...e.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	<Xen-devel@...ts.xen.org>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	<dhillf@...il.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with
 existing use

On 08/22/2013 04:51 AM, Dave Jones wrote:
> On Wed, Aug 21, 2013 at 04:04:54PM -0700, Linus Torvalds wrote:
>  > On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>  > >
>  > > I personally don't see bug here because
>  > >
>  > >  - this swapped page soft dirty bit is set for non-present entries only,
>  > >    never for present ones, just at moment we form swap pte entry
>  > >
>  > >  - i don't find any code which would test for this bit directly without
>  > >    is_swap_pte call
>  > 
>  > Ok, having gone through the places that use swp_*soft_dirty(), I have
>  > to agree. Afaik, it's only ever used on a swap-entry that has (by
>  > definition) the P bit clear. So with or without Xen, I don't see how
>  > it can make any difference.
>  > 
>  > David/Konrad - did you actually see any issues, or was this just from
>  > (mis)reading the code?
> 
> Could this explain what I'm seeing in another thread ?
> https://lkml.org/lkml/2013/8/7/27

Was it caught with CONFIG_MEM_SOFT_DIRTY on or off? In the latter case all new 
bits manipulations are no-op and couldn't cause this.

> 	Dave

Thanks,
Pavel
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