Messages in this thread | | | From | Punit Agrawal <> | Subject | Re: [PATCH v2 1/3] arm64: hugetlb: Fix huge_pte_offset to return poisoned page table entries | Date | Thu, 08 Jun 2017 17:28:31 +0100 |
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Will Deacon <will.deacon@arm.com> writes:
> 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?
Summarising our offline discussion - the semantics of huge_pte_offset() are unclear in terms of when a pointer to (p*d) is returned vs when to return NULL.
As part of enabling contiguous hugepage support[0][1], I have a patch to add a size argument to huge_pte_offset() that can then help disambiguate when we have a valid pte* vs when we have an error (NULL).
I'll separately look at clarifying the semantics of the generic version of huge_pte_offse() and potentially also look at unifying the huge_pte_offset() implementations across the various architectures.
[0] https://lkml.org/lkml/2017/5/24/463 [1] https://www.spinics.net/lists/arm-kernel/msg583367.html
> > Will
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