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Date: Wed, 7 Dec 2022 11:05:13 -0800
From: Sidhartha Kumar <sidhartha.kumar@...cle.com>
To: Mike Kravetz <mike.kravetz@...cle.com>,
Muchun Song <muchun.song@...ux.dev>
Cc: linux-kernel@...r.kernel.org,
Linux Memory Management List <linux-mm@...ck.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Muchun Song <songmuchun@...edance.com>,
Matthew Wilcox <willy@...radead.org>,
Mina Almasry <almasrymina@...gle.com>,
Miaohe Lin <linmiaohe@...wei.com>, hughd@...gle.com,
tsahu@...ux.ibm.com, jhubbard@...dia.com,
David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH mm-unstable v5 01/10] mm: add folio dtor and order setter
functions
On 12/7/22 10:49 AM, Sidhartha Kumar wrote:
> 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@...cle.com> wrote:
>>>>
>>>> On 12/07/22 11:34, Muchun Song wrote:
>>>>>
>>>>>
>>>>>> On Nov 30, 2022, at 06:50, Sidhartha Kumar
>>>>>> <sidhartha.kumar@...cle.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@...cle.com>
>>>>>> Suggested-by: Muchun Song <songmuchun@...edance.com>
>>>>>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.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?
>
>
I will change the comment from referencing
__destory_compound_gigantic_page()
to __destroy_compound_gigantic_folio, although
__prep_compound_gigantic_folio() is another user of
folio_set_compound_order(folio, 0). Should the sentence just be "An
exception is hugetlb, which passes in a zero order"?
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