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 21:25:47 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Jan Beulich <JBeulich@...e.com>,
	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>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: Regression: x86/mm: new _PTE_SWP_SOFT_DIRTY bit conflicts with
 existing use

On Wed, Aug 21, 2013 at 05:56:08PM +0100, David Vrabel wrote:
> > 
> > And I asked David to point me how it happens, because I don't
> > understand at which point pse bit get analized when page is
> > not present.
> 
> As Jan said, we're concerned that the bit was being used on present PTEs
> and not just non-present ones.  From a more careful look at this code
> this does not appear to be the case.
> 
> However, I do find the use of PTE bits in this way somewhat fragile.
> What other potential corner cases might still remain that will require
> further games with PTE bits?

  OK, so this is not a bug finally. The problem is that 2 level pte is
quite small and 7th bit is the only one spare I can use for soft dirty
tracking when page get swapped out. And swap engine is very depending
on pte being non-present, so we are on a safe side.

> FWIW, Xen uses a separate dirty log to track which pages have become
> dirty since the log was last cleared.  Such a dirty log seems more
> efficient than having scan all the PTEs looking for the soft dirty bits
> and then having to scan them all again to clear them (particularly if
> you need multiple passes because the task is still running and
> continuing to dirty pages).
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