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Subject: Re: [PATCH 6/6] x86: Fix stray A/D bit setting into non-present PTEs
To: "Eric W. Biederman" <ebiederm@xmission.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
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Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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        Dave Hansen <dave.hansen@linux.intel.com>
From: Dave Hansen <dave@sr71.net>
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Date: Fri, 1 Jul 2016 08:51:36 -0700
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On 07/01/2016 07:25 AM, Eric W. Biederman wrote:
> Linus Torvalds <torvalds@linux-foundation.org> writes:
>> > On Thu, Jun 30, 2016 at 9:39 PM, Dave Hansen <dave@sr71.net> wrote:
>>> >>
>>> >> 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.
>> >
>> > Yeah, we would need to change pte_none() to mask off D/A, but I think
>> > that might be the only real change needed (other than making sure that
>> > we don't use the bits in the swap entries, I didn't look at that part
>> > at all)
> It looks like __pte_to_swp_entry also needs to be changed to mask out
> those bits when the swap code reads pte entries.  For all of the same
> reasons as pte_none.

I guess that would be nice, but isn't it redundant?

static inline swp_entry_t pte_to_swp_entry(pte_t pte)
{
	...
        arch_entry = __pte_to_swp_entry(pte);
	return swp_entry(__swp_type(arch_entry), __swp_offset(arch_entry));
}

As long as __swp_type() and __swp_offset() don't let A/D through, then
we should be OK.  This site is the only call to __pte_to_swp_entry()
that I can find in the entire codebase.

Or am I missing something?