On Tue 25-07-17 16:41:14, 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 NULL
> in the case of p*d_none() and a pointer to the pte_t for hugepage or
> swap entries.
>
> 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().
hugetlb pte semantic is a disaster and I agree it could see some
cleanup/clarifications but I am quite nervous to see a patchi like this.
How do we check that nothing will get silently broken by this change?
> Signed-off-by: Punit Agrawal <punit.agrawal@xxxxxxx>
> ---
> mm/hugetlb.c | 22 +++++++++++++++++++---
> 1 file changed, 19 insertions(+), 3 deletions(-)
>
> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
> index bc48ee783dd9..72dd1139a8e4 100644
> --- a/mm/hugetlb.c
> +++ b/mm/hugetlb.c
> @@ -4603,6 +4603,13 @@ 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 the entry is p*d_none().
> + */
> pte_t *huge_pte_offset(struct mm_struct *mm,
> unsigned long addr, unsigned long sz)
> {
> @@ -4617,13 +4624,22 @@ 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 (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 (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 */
> --
> 2.11.0
>
--
Michal Hocko
SUSE Labs
