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

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Date:	Thu, 30 Jun 2016 21:39:52 -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 06/30/2016 07:55 PM, Linus Torvalds wrote:
> On Thu, Jun 30, 2016 at 5:12 PM, Dave Hansen <dave@...1.net> wrote:
>> From: Dave Hansen <dave.hansen@...ux.intel.com>
>> The Intel(R) Xeon Phi(TM) Processor x200 Family (codename: Knights
>> Landing) has an erratum where a processor thread setting the Accessed
>> or Dirty bits may not do so atomically against its checks for the
>> Present bit.  This may cause a thread (which is about to page fault)
>> to set A and/or D, even though the Present bit had already been
>> atomically cleared.
> 
> So I don't think your approach is wrong, but I suspect this is
> overkill, and what we should instead just do is to not use the A/D
> bits at all in the swap representation.

We actually don't even use Dirty today.  It's (implicitly) used to
determine pte_none(), but it ends up being masked out for the
swp_offset/type() calculations entirely, much to my surprise.

I think what you suggest will work if we don't consider A/D in
pte_none().  I think there are a bunch of code path where assume that
!pte_present() && !pte_none() means swap.

> The swap-entry representation was a bit tight on 32-bit page table
> entries, but in 64-bit ones, I think we have tons of bits, don't we?
> So we could decide just to not use those two bits on x86.

Yeah, we've definitely got space.  I'll go poke around and make sure
that this works everywhere.  I agree that throwing 32-bit non-PAE under
the bus is definitely worth it here.  Nobody will care about that in a
million years.