On Fri 18-08-17 15:54:15, Punit Agrawal wrote: > When walking the page tables to resolve an address that points to > !p*d_present() entry, huge_pte_offset() returns inconsistent values > depending on the level of page table (PUD or PMD). > > It returns NULL in the case of a PUD entry while in the case of a PMD > entry, it returns a pointer to the page table entry. > > A similar inconsitency exists when handling swap entries - returns NULL > for a PUD entry while a pointer to the pte_t is retured for the PMD entry. > > Update huge_pte_offset() to make the behaviour consistent - return a > pointer to the pte_t for hugepage or swap entries. Only return NULL in > instances where we have a p*d_none() entry and the size parameter > doesn't match the hugepage size at this level of the page table. > > Document the behaviour to clarify the expected behaviour of this function. > This is to set clear semantics for architecture specific implementations > of huge_pte_offset(). > > Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx> > Cc: Catalin Marinas <catalin.marinas@xxxxxxx> > Cc: Naoya Horiguchi <n-horiguchi@xxxxxxxxxxxxx> > Cc: Steve Capper <steve.capper@xxxxxxx> > Cc: Will Deacon <will.deacon@xxxxxxx> > Cc: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> > Cc: Michal Hocko <mhocko@xxxxxxxx> > Cc: Mike Kravetz <mike.kravetz@xxxxxxxxxx> I always thought that the weird semantic is a result of the hugetlb pte sharing. But now that I dug into history it has been added by 02b0ccef903e ("[PATCH] hugetlb: check p?d_present in huge_pte_offset()") for a completely different reason. I suspec the weird semantic just wasn't noticed back then. Anyway, I didn't find any problem with the patch Acked-by: Michal Hocko <mhocko@xxxxxxxx> > --- > > Hi Andrew, > > >From discussions on the arm64 implementation of huge_pte_offset()[0] > we realised that there is benefit from returning a pte_t* in the case > of p*d_none(). > > The fault handling code in hugetlb_fault() can handle p*d_none() > entries and saves an extra round trip to huge_pte_alloc(). Other > callers of huge_pte_offset() should be ok as well. > > Apologies for sending a late update but I thought if we are defining > the semantics, it's worth getting them right. > > Could you please pick this version please? > > Thanks, > Punit > > [0] http://www.spinics.net/lists/linux-mm/msg133699.html > > v2: > > mm/hugetlb.c | 24 +++++++++++++++++++++--- > 1 file changed, 21 insertions(+), 3 deletions(-) > > diff --git a/mm/hugetlb.c b/mm/hugetlb.c > index 31e207cb399b..1d54a131bdd5 100644 > --- a/mm/hugetlb.c > +++ b/mm/hugetlb.c > @@ -4600,6 +4600,15 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, > return pte; > } > > +/* > + * huge_pte_offset() - Walk the page table to resolve the hugepage > + * entry at address @addr > + * > + * Return: Pointer to page table or swap entry (PUD or PMD) for > + * address @addr, or NULL if a p*d_none() entry is encountered and the > + * size @sz doesn't match the hugepage size at this level of the page > + * table. > + */ > pte_t *huge_pte_offset(struct mm_struct *mm, > unsigned long addr, unsigned long sz) > { > @@ -4614,13 +4623,22 @@ pte_t *huge_pte_offset(struct mm_struct *mm, > p4d = p4d_offset(pgd, addr); > if (!p4d_present(*p4d)) > return NULL; > + > pud = pud_offset(p4d, addr); > - if (!pud_present(*pud)) > + if (sz != PUD_SIZE && pud_none(*pud)) > return NULL; > - if (pud_huge(*pud)) > + /* hugepage or swap? */ > + if (pud_huge(*pud) || !pud_present(*pud)) > return (pte_t *)pud; > + > pmd = pmd_offset(pud, addr); > - return (pte_t *) pmd; > + if (sz != PMD_SIZE && pmd_none(*pmd)) > + return NULL; > + /* hugepage or swap? */ > + if (pmd_huge(*pmd) || !pmd_present(*pmd)) > + return (pte_t *)pmd; > + > + return NULL; > } > > #endif /* CONFIG_ARCH_WANT_GENERAL_HUGETLB */ > -- > 2.13.2 > -- Michal Hocko SUSE Labs