Re: x86/mm/pageattr: Code without effect?

From: Andy Whitcroft
Date: Mon Apr 08 2013 - 10:53:39 EST

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);

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