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

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Date:   Wed, 7 Dec 2022 12:11:56 +0800
From:   Muchun Song <muchun.song@...ux.dev>
To:     Mike Kravetz <mike.kravetz@...cle.com>
Cc:     Sidhartha Kumar <sidhartha.kumar@...cle.com>,
        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 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
}

Thanks.