linux-kernel - Re: [PATCH mm-unstable] mm: move folio_set_compound

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Date:   Wed, 14 Dec 2022 12:35:23 -0800
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     John Hubbard <jhubbard@...dia.com>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Cc:     akpm@...ux-foundation.org, songmuchun@...edance.com,
        mike.kravetz@...cle.com, willy@...radead.org
Subject: Re: [PATCH mm-unstable] mm: move folio_set_compound_order() to
 mm/internal.h

On 12/14/22 12:43 AM, John Hubbard wrote:
> On 12/13/22 13:20, Sidhartha Kumar wrote:
>> folio_set_compound_order() is moved to an mm-internal location so external
>> folio users cannot misuse this function. Change the name of the function
>> to folio_set_order() and use WARN_ON_ONCE() rather than BUG_ON. Also,
>> handle the case if a non-large folio is passed and add clarifying comments
>> to the function.
>>
>> Link: https://lore.kernel.org/lkml/20221207223731.32784-1-sidhartha.kumar@oracle.com/T/
>> Fixes: 9fd330582b2f ("mm: add folio dtor and order setter functions")
>>
>> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@...cle.com>
>> Suggested-by: Mike Kravetz <mike.kravetz@...cle.com>
>> Suggested-by: Muchun Song <songmuchun@...edance.com>
>> Suggested-by: Matthew Wilcox <willy@...radead.org>
>> Suggested-by: John Hubbard <jhubbard@...dia.com>
>> ---
>>   include/linux/mm.h | 16 ----------------
>>   mm/hugetlb.c       |  6 +++---
>>   mm/internal.h      | 21 +++++++++++++++++++++
>>   3 files changed, 24 insertions(+), 19 deletions(-)
> 
> I think this looks good. One small question below.
> 
>>
>> diff --git a/include/linux/mm.h b/include/linux/mm.h
>> index 7dc376052d40..300d92d2b49d 100644
>> --- a/include/linux/mm.h
>> +++ b/include/linux/mm.h
>> @@ -1019,22 +1019,6 @@ static inline void set_compound_order(struct page *page, unsigned int order)
>>   #endif
>>   }
>>   
>> -/*
>> - * folio_set_compound_order is generally passed a non-zero order to
>> - * initialize a large folio.  However, hugetlb code abuses this by
>> - * passing in zero when 'dissolving' a large folio.
>> - */
>> -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;
>> -#endif
>> -}
>> -
>>   /* Returns the number of pages in this potentially compound page. */
>>   static inline unsigned long compound_nr(struct page *page)
>>   {
>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>> index 7cdbcc22587b..810e840bb4f1 100644
>> --- a/mm/hugetlb.c
>> +++ b/mm/hugetlb.c
>> @@ -1344,7 +1344,7 @@ static void __destroy_compound_gigantic_folio(struct folio *folio,
>>   			set_page_refcounted(p);
>>   	}
>>   
>> -	folio_set_compound_order(folio, 0);
>> +	folio_set_order(folio, 0);
>>   	__folio_clear_head(folio);
>>   }
>>   
>> @@ -1808,7 +1808,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>>   	__folio_clear_reserved(folio);
>>   	__folio_set_head(folio);
>>   	/* we rely on prep_new_hugetlb_folio to set the destructor */
>> -	folio_set_compound_order(folio, order);
>> +	folio_set_order(folio, order);
>>   	for (i = 0; i < nr_pages; i++) {
>>   		p = folio_page(folio, i);
>>   
>> @@ -1872,7 +1872,7 @@ static bool __prep_compound_gigantic_folio(struct folio *folio,
>>   		p = folio_page(folio, j);
>>   		__ClearPageReserved(p);
>>   	}
>> -	folio_set_compound_order(folio, 0);
>> +	folio_set_order(folio, 0);
>>   	__folio_clear_head(folio);
>>   	return false;
>>   }
>> diff --git a/mm/internal.h b/mm/internal.h
>> index bcf75a8b032d..829b6a60ceb7 100644
>> --- a/mm/internal.h
>> +++ b/mm/internal.h
>> @@ -378,6 +378,27 @@ extern void *memmap_alloc(phys_addr_t size, phys_addr_t align,
>>   int split_free_page(struct page *free_page,
>>   			unsigned int order, unsigned long split_pfn_offset);
>>   
>> +/*
>> + * This will have no effect, other than possibly generating a warning, if the
>> + * caller passes in a non-large folio.
>> + */
>> +static inline void folio_set_order(struct folio *folio, unsigned int order)
>> +{
>> +	if (!folio_test_large(folio)) {
>> +		WARN_ON_ONCE(order);
>> +		return;
>> +	}
> 
> Would it be better to do this (below)? I'm not sure of the value of
> warning on "order"--it's a little odd and unexplained and doesn't really
> do anything more helpful than simply warning about what why the code is
> failing, which is really about !large, rather than order. Unless I'm
> missing something?
> 
> 	if (WARN_ON_ONCE(!folio_test_large(folio)))
> 		return;
>I agree that warning this way is clearer, I will change in a v2.

> Sorry to drive you crazy over nits. This is the last one from me. :)
> 
No worries, I appreciate the feedback.

Thanks,
Sidhartha Kumar

> thanks,