linux-kernel - Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key faults

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Date:	Sat, 26 Sep 2015 08:20:23 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Dave Hansen <dave@...1.net>
Cc:	x86@...nel.org, linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key
 faults


* Dave Hansen <dave@...1.net> wrote:

> On 09/25/2015 12:11 AM, Ingo Molnar wrote:
> >>> > > Btw., how does pkey support interact with hugepages?
> >> > 
> >> > Surprisingly little.  I've made sure that everything works with huge pages and 
> >> > that the (huge) PTEs and VMAs get set up correctly, but I'm not sure I had to 
> >> > touch the huge page code at all.  I have test code to ensure that it works the 
> >> > same as with small pages, but everything worked pretty naturally.
> > Yeah, so the reason I'm asking about expectations is that this code:
> > 
> > +       follow_ret = follow_pte(tsk->mm, address, &ptep, &ptl);
> > +       if (!follow_ret) {
> > +               /*
> > +                * On a successful follow, make sure to
> > +                * drop the lock.
> > +                */
> > +               pte = *ptep;
> > +               pte_unmap_unlock(ptep, ptl);
> > +               ret = pte_pkey(pte);
> > 
> > is visibly hugepage-unsafe: if a vma is hugepage mapped, there are no ptes, only 
> > pmds - and the protection key index lives in the pmd. We don't seem to recover 
> > that information properly.
> 
> You got me on this one.  I assumed that follow_pte() handled huge pages.
>  It does not.
> 
> But, the code still worked.  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?

> 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.

Thanks,

	Ingo
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