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From: Andy Lutomirski <luto@amacapital.net>
Date: Wed, 21 Aug 2013 12:07:13 -0700
Message-ID: <CALCETrWe-BH-ope+bdnW0jJy6EGcEGeT4xA=SZT4xZQO+MMTMA@mail.gmail.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with
 existing use
To: Cyrill Gorcunov <gorcunov@gmail.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>, David Vrabel <david.vrabel@citrix.com>,
        Jan Beulich <JBeulich@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Xen-devel@lists.xen.org, Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Pavel Emelyanov <xemul@parallels.com>, Ingo Molnar <mingo@redhat.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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On Wed, Aug 21, 2013 at 12:03 PM, Cyrill Gorcunov <gorcunov@gmail.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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