linux-kernel - Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs

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Date:	Fri, 1 Jul 2016 09:14:13 -0700
From:	Dave Hansen <dave@...1.net>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	the arch/x86 maintainers <x86@...nel.org>,
	linux-mm <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>,
	Andi Kleen <ak@...ux.intel.com>,
	Michal Hocko <mhocko@...e.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>
Subject: Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs

On 07/01/2016 09:07 AM, Linus Torvalds wrote:
> But I also started worrying about us just losing sight of the dirty
> bit in particular. It's not enough that we ignore the dirty bit - we'd
> still want to make sure that the underlying backing page gets marked
> dirty, even if the CPU is buggy and ends doing it "delayed" after
> we've already unmapped the page.
> 
> So I get this feeling that we may need a fair chunk of your
> patch-series anyway.

As I understand it, the erratum only affects a thread which is about to
page fault.  The write associated with the dirty bit being set never
actually gets executed.  So, the bit really *is* stray and isn't
something we need to preserve.

Otherwise, we'd be really screwed because we couldn't ever simply clear it.