Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1233155

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

Show all headers | View raw

* Dave Hansen <dave@sr71.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
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/

Back to linux.kernel | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Ingo Molnar <mingo@kernel.org> - 2015-09-24 11:30 +0200
  Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Ingo Molnar <mingo@kernel.org> - 2015-09-24 11:40 +0200
    Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Dave Hansen <dave@sr71.net> - 2015-09-24 19:50 +0200
      Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Ingo Molnar <mingo@kernel.org> - 2015-09-25 09:20 +0200
        Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Dave Hansen <dave@sr71.net> - 2015-09-26 01:20 +0200
          Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Ingo Molnar <mingo@kernel.org> - 2015-09-26 08:30 +0200
            Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Dave Hansen <dave@sr71.net> - 2015-09-28 00:50 +0200
              Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Ingo Molnar <mingo@kernel.org> - 2015-09-28 08:00 +0200
  Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Dave Hansen <dave@sr71.net> - 2015-09-24 19:20 +0200
    Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Christian Borntraeger <borntraeger@de.ibm.com> - 2015-09-28 21:30 +0200
      Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key  faults Dave Hansen <dave@sr71.net> - 2015-09-28 21:40 +0200

csiph-web