Re: [PATCH v2] mm/hugetlb: fix hugetlb vs. core-mm PT locking

David Hildenbrand
2024-07-30T21:19:42Z

On 30.07.24 23:17, James Houghton wrote:
> On Tue, Jul 30, 2024 at 2:07 PM David Hildenbrand <david@redhat.com> wrote:
>>
>> On 30.07.24 23:00, David Hildenbrand wrote:
>>> On 30.07.24 22:43, James Houghton wrote:
>>>> On Tue, Jul 30, 2024 at 1:03 PM David Hildenbrand <david@redhat.com> wrote:
>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>> index b100df8cb5857..1b1f40ff00b7d 100644
>>>>> --- a/include/linux/mm.h
>>>>> +++ b/include/linux/mm.h
>>>>> @@ -2926,6 +2926,12 @@ static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
>>>>> return ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
>>>>> }
>>>>>
>>>>> +static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
>>>>> +{
>>>>> + BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE));
>>>>> + return ptlock_ptr(virt_to_ptdesc(pte));
>>>>
>>>> Hi David,
>>>>
>>>
>>> Hi!
>>>
>>>> Small question: ptep_lockptr() does not handle the case where the size
>>>> of the PTE table is larger than PAGE_SIZE, but pmd_lockptr() does.
>>>
>>> I thought I convinced myself that leaf page tables are always single
>>> pages and had a comment in v1.
>>>
>>> But now I have to double-check again, and staring at
>>> pagetable_pte_ctor() callers I am left confused.
>>>
>>> It certainly sounds more future proof to just align the pointer down to
>>> the start of the PTE table like pmd_lockptr() would.
>>>
>>>> IIUC, for pte_lockptr() and ptep_lockptr() to return the same result
>>>> in this case, ptep_lockptr() should be doing the masking that
>>>> pmd_lockptr() is doing. Are you sure that you don't need to be doing
>>>> it? (Or maybe I am misunderstanding something.)
>>>
>>> It's a valid concern even if it would not be required. But I'm afraid I
>>> won't dig into the details and simply do the alignment in a v3.
>>
>> To be precise, the following on top:
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 1b1f40ff00b7d..f6c7fe8f5746f 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -2926,10 +2926,22 @@ static inline spinlock_t *pte_lockptr(struct mm_struct *mm, pmd_t *pmd)
>> return ptlock_ptr(page_ptdesc(pmd_page(*pmd)));
>> }
>>
>> -static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
>> +static inline struct page *ptep_pgtable_page(pte_t *pte)
>> {
>> + unsigned long mask = ~(PTRS_PER_PTE * sizeof(pte_t) - 1);
>> +
>> BUILD_BUG_ON(IS_ENABLED(CONFIG_HIGHPTE));
>> - return ptlock_ptr(virt_to_ptdesc(pte));
>> + return virt_to_page((void *)((unsigned long)pte & mask));
>> +}
>> +
>> +static inline struct ptdesc *ptep_ptdesc(pte_t *pte)
>> +{
>> + return page_ptdesc(ptep_pgtable_page(pte));
>> +}
>> +
>> +static inline spinlock_t *ptep_lockptr(struct mm_struct *mm, pte_t *pte)
>> +{
>> + return ptlock_ptr(ptep_ptdesc(pte));
>> }
>
> Thanks! That looks right to me. Feel free to add
>
> Reviewed-by: James Houghton <jthoughton@google.com>

Thanks for the review, will send a v3 tomorrow after having wasted more
valuable life time setting up the ARM environment again ... :)

--
Cheers,

David / dhildenb