linux-kernel - Re: x86/mm/pageattr: Code without effect?

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Date:	Mon, 8 Apr 2013 15:53:31 +0100
From:	Andy Whitcroft <apw@...onical.com>
To:	Andrea Arcangeli <aarcange@...hat.com>
Cc:	Borislav Petkov <bp@...en8.de>,
	Stefan Bader <stefan.bader@...onical.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>
Subject: Re: x86/mm/pageattr: Code without effect?

On Sat, Apr 06, 2013 at 04:58:04PM +0200, Andrea Arcangeli wrote:

> You're right, so this location clearly didn't trigger the problem so I
> didn't notice the noop here. I only exercised the fix in the other
> locations of the file that had the same problem.
> 
> It was a noop, so it really couldn't hurt but the below change should
> activate the fix there too. On the same lines, there was a superfluous
> initialization of new_prot too which I cleaned up.

Although the new code is essentially noop, the other part of the change
in try_preserve_large_page() moves the canon_pgprot() up above the
static_protections() incantation which replaces its value, thus we lose
the effect of that on the protection bits.  I suspect this only affects
older CPUs (?) but I do think there is a negative semantic change here:

+       new_prot = canon_pgprot(new_prot);
[...]
        new_prot = static_protections(req_prot, address, pfn);
[...]
-               new_pte = pfn_pte(pte_pfn(old_pte), canon_pgprot(new_prot));
+               new_pte = pfn_pte(pte_pfn(old_pte), new_prot);

-apw
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