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

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Date:	Wed, 21 Aug 2013 12:07:13 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Jan Beulich <JBeulich@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Xen-devel@...ts.xen.org,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Ingo Molnar <mingo@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with
 existing use

On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Wed, Aug 21, 2013 at 08:50:26PM +0200, H. Peter Anvin wrote:
>> But is there a manifest bug or not?  What is the deal with Xen?
>>
>
> 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
>
> but the use of paw bit itself is confusing, so I'm working on patch which
> won't use it. Again, if someone knows where exactly access to pse bit when
> pte keeps swap entry may happen (for any purpose other than dirty page
> tracking) please share.

I doubt that there are cacheability issues here, since the swap type
already overlaps with PWT and PCD.

What happens if the page being swapped is a THP page?  (I have no idea
how, or even if, this works, but presumably PSE is important.)

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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