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 11:03:05 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Jan Beulich <JBeulich@...e.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 Thu, Aug 22, 2013 at 07:56:26AM +0100, Jan Beulich wrote:
> >>> On 21.08.13 at 18:19, Cyrill Gorcunov <gorcunov@...il.com> wrote:
> > On Wed, Aug 21, 2013 at 05:03:13PM +0100, Jan Beulich wrote:
> >> > 
> >> > Only to non-present ptes, as far as I know.
> >> 
> >> That's not really any guarantee. And the accessor functions also
> >> don't check that they'd be used on non-present PTEs only.
> > 
> > Wait. This _PAGE_SWP_SOFT_DIRTY bit (which is in real PSE bit) assigned
> > in only one place -- in try_to_unmap_one(). The PTE get non-present then
> > and consists of swap entry format. I don't see any accessor to such entry
> > without testing if it's swap entry or pte-none. What I'm missing?
> 
> Fact is that this
> 
> static inline pte_t pte_swp_mksoft_dirty(pte_t pte)
> {
> 	return pte_set_flags(pte, _PAGE_SWP_SOFT_DIRTY);
> }
> 
> has no checking whatsoever that the PTE being modified is a
> non-present one, not even in any of the debugging modes. It
> would be a different thing if the above acted on a swp_entry_t.
> 
> The fact that there currently may be just a single call site (where
> the caller guarantees the non-present state) is no guarantee that
> in the future another one won't appear, and then result in very
> hard to debug problems.

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);
}
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