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Subject: Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key
 faults
To: Ingo Molnar <mingo@kernel.org>
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Cc: x86@kernel.org, linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Peter Zijlstra <a.p.zijlstra@chello.nl>,
        Thomas Gleixner <tglx@linutronix.de>
From: Dave Hansen <dave@sr71.net>
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Date: Sun, 27 Sep 2015 15:39:58 -0700
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On 09/25/2015 11:20 PM, Ingo Molnar wrote:
> * Dave Hansen <dave@sr71.net> wrote:
...
>> Since follow_pte() fails for all huge
>> pages, it just falls back to pulling the protection key out of the VMA,
>> which _does_ work for huge pages.
> 
> That might be true for explicit hugetlb vmas, but what about transparent hugepages 
> that can show up in regular vmas?

All PTEs (large or small) established under a given VMA have the same
protection key.  Any change in protection key for a range will either
change or split the VMA.

So I think it's safe to rely on the VMA entirely.  Well, as least as
safe as the PTE.  It's definitely a wee bit racy, which I'll elaborate
on when I repost the patches.

>> I've actually removed the PTE walking and I just now use the VMA directly.  I 
>> don't see a ton of additional value from walking the page tables when we can get 
>> what we need from the VMA.
> 
> That's actually good, because it's also cheap, especially if we can get rid of the 
> extra find_vma().
> 
> and we (thankfully) have no non-linear vmas to worry about anymore.

Yep.