Catalin Marinas <catalin.marinas@xxxxxxx> writes:
> Hi Punit,
>
> On Mon, Jul 24, 2017 at 06:33:17PM +0100, Punit Agrawal wrote:
>> When walking the page tables to resolve an address that points to
>> !present_p*d() entry, huge_pte_offset() returns inconsistent values
>> depending on the level of page table (PUD or PMD).
>>
>> In the case of a PUD entry, it returns NULL while in the case of a PMD
>> entry, it returns a pointer to the page table entry.
>>
>> Make huge_pte_offset() consistent by always returning NULL on
>> encountering a !present_p*d() entry. Document the behaviour to clarify
>> the expected semantics of this function.
>
> Nitpick: "p*d_present" instead of "present_p*d".
Thanks for spotting. Fixed both the instances locally.
>
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index bc48ee783dd9..686eb6fa9eb1 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 entry (PUD or PMD) for address @addr
>> + * or NULL if the entry is not present.
>> + */
>> pte_t *huge_pte_offset(struct mm_struct *mm,
>> unsigned long addr, unsigned long sz)
>> {
>> @@ -4617,13 +4624,20 @@ 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))
>> return NULL;
>> if (pud_huge(*pud))
>> return (pte_t *)pud;
>> +
>> pmd = pmd_offset(pud, addr);
>> - return (pte_t *) pmd;
>> + if (!pmd_present(*pmd))
>> + return NULL;
>
> This breaks the current behaviour for swap entries in the pmd (for pud
> is already broken but maybe no-one uses them). It is fixed in the
> subsequent patch together with the pud but the series is no longer
> bisectable. Maybe it's better if you fold the two patches together (or
> change the order, though I'm not sure how readable it is).
I missed the change in behaviour for pmd swap entries. I'll squash the
two patches and re-post.
Thanks for the review.
