* Dave Hansen <dave@xxxxxxxx> 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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