Messages in this thread | | | Subject | Re: [PATCH 10/26] x86, pkeys: notify userspace about protection key faults | From | Dave Hansen <> | Date | Fri, 25 Sep 2015 16:18:56 -0700 |
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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.
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.
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