Re: [PATCH 1/2] powerpc/mm: Fix crashes with PUD level hugetlb config

[Ram Pai <linuxram@us.ibm.com>]

On Sat, Feb 10, 2018 at 03:17:02PM +0530, Aneesh Kumar K.V wrote:
> Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com> writes:
> 
> > "Aneesh Kumar K.V" <aneesh.kumar@linux.vnet.ibm.com> writes:
> >
> >> To support memory keys, we moved the hash pte slot information to the second
> >> half of the page table. This was ok with PTE entries at level 4 and level 3.
> >> We already allocate larger page table pages at those level to accomodate extra
> >> details. For level 4 we already have the extra space which was used to track
> >> 4k hash page table entry details and at pmd level the extra space was allocated
> >> to track the THP details.
> >>
> >> With hugetlbfs PTE, we used this extra space at the PMD level to store the
> >> slot details. But we also support hugetlbfs PTE at PUD leve and PUD level page
> >> didn't allocate extra space. This resulted in memory corruption.
> >>
> >> Fix this by allocating extra space at PUD level when HUGETLB is enabled. We
> >> may need further changes to allocate larger space at PMD level when we enable
> >> HUGETLB. That will be done in next patch.
> >>
> >> Fixes:bf9a95f9a6481bc6e(" powerpc: Free up four 64K PTE bits in 64K backed HPTE pages")
> >>
> >> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> >
> > Another fix, I still get random memory corruption with hugetlb test with
> > 16G hugepage config.
> 
> Another one. I am not sure whether we really want this in this form. But
> with this tests are running fine.
> 
> -aneesh
> 
> commit 658fe8c310a913e69e5bc9a40d4c28a3b88d5c08
> Author: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> Date:   Sat Feb 10 13:17:34 2018 +0530
> 
>     powerpc/mm/hash64: memset the pagetable pages on allocation.
>     
>     Now that we are using second half of the table to store slot details and we
>     don't clear them in the huge_pte_get_and_clear, we need to make sure we zero
>     out the range on allocation. This done some extra work because the first half
>     of the table is cleared by huge_pte_get_and_clear and memset in this patch
>     zero-out the full table page.
>     
>     We need to do this for pgd and pud because both get allocated from the same slab
>     cache.

Do we need to zero pgd aswell to resolve your corruption? pud is not
sufficient? Or was it done to avoid issues in the future in case pgd
is used as the leaf; possibly for Terra_huge_pages?

RP