Messages in this thread | | | Date | Wed, 7 Jun 2017 17:54:30 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH v2 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries |
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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().
At a quick look, there are a few other places where not returning NULL has some other effects (though I don't think any of them are fatal):
- copy_huge_tlb_page_range() - unnecessary allocation of a destination pud
- huge_pmd_share() - do we actually need the subsequent get_page()?
- page_vma_mapped_walk() - it even has a comment: "when pud is not present, pte will be NULL". Now, that's no longer true with swap entries but we'd never return NULL for a pud_none() case
Current code behaviour is to return NULL when !p*d_present(). We are slightly relaxing this for swap entries while still returning NULL for the p*d_none() case but I wouldn't go that far as to never return NULL here.
-- Catalin
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