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 12:33:57 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Cyrill Gorcunov" <gorcunov@...il.com>
Cc:	"Andy Lutomirski" <luto@...capital.net>,
	"David Vrabel" <david.vrabel@...rix.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>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts
 with existing use

>>> On 22.08.13 at 13:27, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> On Thu, Aug 22, 2013 at 08:27:45AM +0100, Jan Beulich wrote:
>> >>> On 22.08.13 at 09:03, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>> > Ok, how about this?
>> > 
>> > static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
>> > {
>> > 	BUG_ON(pte_present(pte));
>> > 	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
>> > }
>> 
>> Sure, fine with me. Perhaps VM_BUG_ON() or some other similar
>> construct limiting the scope when any extra code gets generated
>> would do too.
> 
> Sorry for delay, the patch is below.
> 
>> 
>> But as said, even better would perhaps be to have it act on a
>> swp_entry_t.
> 
> swp_entry_t is too small already to keep additional status bit,
> unfortunately.
> ---
> From: Cyrill Gorcunov <gorcunov@...il.com>
> Subject: [PATCH] mm: Make sure _PAGE_SWP_SOFT_DIRTY bit is not set on 
> present pte
> 
> _PAGE_SOFT_DIRTY bit should never be set on present pte so add
> VM_BUG_ON to catch any potential future abuse.
> 
> Also add a comment on _PAGE_SWP_SOFT_DIRTY definition explaining
> scope of its usage.
> 
> Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>

Acked-by: Jan Beulich <jbeulich@...e.com>

Thanks, Jan

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