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

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Message-ID: <71a3ac45-a091-4e5a-aa04-68af9219e30f@email.android.com>
Date:	Wed, 21 Aug 2013 16:58:39 +0200
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Jan Beulich <JBeulich@...e.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Cyrill Gorcunov <gorcunov@...il.com>
CC:	Andy Lutomirski <luto@...capital.net>,
	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

Only WB pages should be swappable, but even so, the cacheability should be in the vma.

Jan Beulich <JBeulich@...e.com> wrote:
>>>> On 21.08.13 at 16:12, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>> On Wed, Aug 21, 2013 at 02:48:20PM +0100, David Vrabel wrote:
>>> All,
>>> 
>>> 179ef71c (mm: save soft-dirty bits on swapped pages) introduces a
>new
>>> PTE bit on x86 _PTE_SWP_SOFT_DIRTY which has the same value as
>_PTE_PSE
>>> and _PTE_PAT.
>>> 
>>> With a Xen PV guest, the use of the _PTE_PAT will result in the page
>>> having unexpected cachability which will introduce a range of subtle
>>> performance and correctness issues.  Xen programs the entry 4 in the
>PAT
>>> table with WC so a page that was previously WB will end up as WC.
>>> 
>> 
>> David, could you please explain, Xen keeps and analyze _PTE_PAT bit
>> for ptes which are not present?
>
>No, the problem isn't with not-present PTEs (i.e. swap entries),
>but with present ones - the same bit (7) is being used for both,
>according to this comment:
>
>/*
> * Tracking soft dirty bit when a page goes to a swap is tricky.
> * We need a bit which can be stored in pte _and_ not conflict
> * with swap entry format. On x86 bits 6 and 7 are *not* involved
> * into swap entry computation, but bit 6 is used for nonlinear
> * file mapping, so we borrow bit 7 for soft dirty tracking.
> */
>
>Or are you telling me that the comment is misleading (at least me),
>and this applies only to not-present PTEs? And even then - where
>would the value of the original PAT bit be stored while swapped
>out (or is it impossible - now and forever - for WC pages to get
>swapped)?
>
>Jan

-- 
Sent from my mobile phone. Please excuse brevity and lack of formatting.
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