Re: [PATCH mm-unstable v5 01/10] mm: add folio dtor and order setter functions

[Sidhartha Kumar <sidhartha.kumar@oracle.com>]

On 12/7/22 10:12 AM, Mike Kravetz wrote:
> On 12/07/22 12:11, Muchun Song wrote:
>>
>>
>>> On Dec 7, 2022, at 11:42, Mike Kravetz <mike.kravetz@oracle.com> wrote:
>>>
>>> On 12/07/22 11:34, Muchun Song wrote:
>>>>
>>>>
>>>>> On Nov 30, 2022, at 06:50, Sidhartha Kumar <sidhartha.kumar@oracle.com> wrote:
>>>>>
>>>>> Add folio equivalents for set_compound_order() and set_compound_page_dtor().
>>>>>
>>>>> Also remove extra new-lines introduced by mm/hugetlb: convert
>>>>> move_hugetlb_state() to folios and mm/hugetlb_cgroup: convert
>>>>> hugetlb_cgroup_uncharge_page() to folios.
>>>>>
>>>>> Suggested-by: Mike Kravetz <mike.kravetz@oracle.com>
>>>>> Suggested-by: Muchun Song <songmuchun@bytedance.com>
>>>>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
>>>>> ---
>>>>> include/linux/mm.h | 16 ++++++++++++++++
>>>>> mm/hugetlb.c       |  4 +---
>>>>> 2 files changed, 17 insertions(+), 3 deletions(-)
>>>>>
>>>>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>>>>> index a48c5ad16a5e..2bdef8a5298a 100644
>>>>> --- a/include/linux/mm.h
>>>>> +++ b/include/linux/mm.h
>>>>> @@ -972,6 +972,13 @@ static inline void set_compound_page_dtor(struct page *page,
>>>>> page[1].compound_dtor = compound_dtor;
>>>>> }
>>>>>
>>>>> +static inline void folio_set_compound_dtor(struct folio *folio,
>>>>> + enum compound_dtor_id compound_dtor)
>>>>> +{
>>>>> + VM_BUG_ON_FOLIO(compound_dtor >= NR_COMPOUND_DTORS, folio);
>>>>> + folio->_folio_dtor = compound_dtor;
>>>>> +}
>>>>> +
>>>>> void destroy_large_folio(struct folio *folio);
>>>>>
>>>>> static inline int head_compound_pincount(struct page *head)
>>>>> @@ -987,6 +994,15 @@ static inline void set_compound_order(struct page *page, unsigned int order)
>>>>> #endif
>>>>> }
>>>>>
>>>>> +static inline void folio_set_compound_order(struct folio *folio,
>>>>> + unsigned int order)
>>>>> +{
>>>>> + folio->_folio_order = order;
>>>>> +#ifdef CONFIG_64BIT
>>>>> + folio->_folio_nr_pages = order ? 1U << order : 0;
>>>>
>>>> It seems that you think the user could pass 0 to order. However,
>>>> ->_folio_nr_pages and ->_folio_order fields are invalid for order-0 pages.
>>>> You should not touch it. So this should be:
>>>>
>>>> static inline void folio_set_compound_order(struct folio *folio,
>>>>      unsigned int order)
>>>> {
>>>> 	if (!folio_test_large(folio))
>>>> 		return;
>>>>
>>>> 	folio->_folio_order = order;
>>>> #ifdef CONFIG_64BIT
>>>> 	folio->_folio_nr_pages = 1U << order;
>>>> #endif
>>>> }
>>>
>>> I believe this was changed to accommodate the code in
>>> __destroy_compound_gigantic_page().  It is used in a subsequent patch.
>>> Here is the v6.0 version of the routine.
>>
>> Thanks for your clarification.
>>
>>>
>>> static void __destroy_compound_gigantic_page(struct page *page,
>>> unsigned int order, bool demote)
>>> {
>>> 	int i;
>>> 	int nr_pages = 1 << order;
>>> 	struct page *p = page + 1;
>>>
>>> 	atomic_set(compound_mapcount_ptr(page), 0);
>>> 	atomic_set(compound_pincount_ptr(page), 0);
>>>
>>> 	for (i = 1; i < nr_pages; i++, p = mem_map_next(p, page, i)) {
>>> 		p->mapping = NULL;
>>> 		clear_compound_head(p);
>>> 		if (!demote)
>>> 			set_page_refcounted(p);
>>> 	}
>>>
>>> 	set_compound_order(page, 0);
>>> #ifdef CONFIG_64BIT
>>> 	page[1].compound_nr = 0;
>>> #endif
>>> 	__ClearPageHead(page);
>>> }
>>>
>>>
>>> Might have been better to change this set_compound_order call to
>>> folio_set_compound_order in this patch.
>>>
>>
>> Agree. It has confused me a lot. I suggest changing the code to the
>> followings. The folio_test_large() check is still to avoid unexpected
>> users for OOB.
>>
>> static inline void folio_set_compound_order(struct folio *folio,
>> 					    unsigned int order)
>> {
>> 	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>> 	// or
>> 	// if (!folio_test_large(folio))
>> 	// 	return;
>>
>> 	folio->_folio_order = order;
>> #ifdef CONFIG_64BIT
>> 	folio->_folio_nr_pages = order ? 1U << order : 0;
>> #endif
>> }
> 
> I think the VM_BUG_ON_FOLIO is appropriate as it would at least flag
> data corruption.
> 
As Mike pointed out, my intention with supporting the 0 case was to 
cleanup the __destroy_compound_gigantic_page code by moving the ifdef 
CONFIG_64BIT lines to folio_set_compound_order(). I'll add the 
VM_BUG_ON_FOLIO line as well as a comment to make it clear it is not 
normally supported.

> Thinking about this some more, it seems that hugetlb is the only caller
> that abuses folio_set_compound_order (and previously set_compound_order)
> by passing in a zero order.  Since it is unlikely that anyone knows of
> this abuse, it might be good to add a comment to the routine to note
> why it handles the zero case.  This might help prevent changes which
> would potentially break hugetlb.

+/*
+ * _folio_nr_pages and _folio_order are invalid for
+ * order-zero pages. An exception is hugetlb, which passes
+ * in a zero order in __destroy_compound_gigantic_page().
+ */
  static inline void folio_set_compound_order(struct folio *folio,
                 unsigned int order)
  {
+       VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
+
         folio->_folio_order = order;
  #ifdef CONFIG_64BIT
         folio->_folio_nr_pages = order ? 1U << order : 0;

Does this comment work?