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    SubjectRe: [PATCH v2 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries
    On Wed, Jun 07, 2017 at 04:32:28PM +0100, Punit Agrawal wrote:
    > Will Deacon <will.deacon@arm.com> writes:
    >
    > > On Wed, Jun 07, 2017 at 03:30:37PM +0100, Catalin Marinas wrote:
    > >> On Wed, Jun 07, 2017 at 02:47:32PM +0100, Will Deacon wrote:
    > >> > On Wed, May 17, 2017 at 04:23:34PM +0100, Punit Agrawal wrote:
    > >> > > --- a/arch/arm64/mm/hugetlbpage.c
    > >> > > +++ b/arch/arm64/mm/hugetlbpage.c
    > >> > > @@ -136,36 +136,27 @@ pte_t *huge_pte_offset(struct mm_struct *mm, unsigned long addr)
    > >> > > {
    > >> > > pgd_t *pgd;
    > >> > > pud_t *pud;
    > >> > > - pmd_t *pmd = NULL;
    > >> > > - pte_t *pte = NULL;
    > >> > > + pmd_t *pmd;
    > >> > >
    > >> > > pgd = pgd_offset(mm, addr);
    > >> > > pr_debug("%s: addr:0x%lx pgd:%p\n", __func__, addr, pgd);
    > >> > > if (!pgd_present(*pgd))
    > >> > > return NULL;
    > >> > > +
    > >> > > pud = pud_offset(pgd, addr);
    > >> > > - if (!pud_present(*pud))
    > >> > > + if (pud_none(*pud))
    > >> > > return NULL;
    > >> >
    > >> > Do you actually need this special case?
    > >> >
    > >> > > -
    > >> > > - if (pud_huge(*pud))
    > >> > > + /* swap or huge page */
    > >> > > + if (!pud_present(*pud) || pud_huge(*pud))
    > >> >
    > >> > ... couldn't you just add a '|| pud_none(*pud)' in here?
    > >> >
    >
    > I think an earlier version took this approach but...
    >
    > >> > > return (pte_t *)pud;
    > >>
    > >> But then you no longer return NULL if *pud == 0.
    > >
    > > Does that actually matter? The bits of hugetlb code I looked at will
    > > deferenced the returned pud and handle the huge_pte_none case correctly.
    >
    > For hugetlb fault handling (hugetlb_fault()), returning NULL vs pointer
    > to the pud/pmd results in different behaviour. If we return the pud when
    > pud_none(), then we lose the resulting hugepage size check we get from
    > huge_pte_alloc().

    Ok, so does that mean that many of the huge_pte_none checks in mm/hugetlb.c
    that operate on a huge_ptep_get of non-NULL output from huge_pte_offset are
    actually redundant?

    Will

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