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

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Date:   Thu, 8 Dec 2022 14:12:18 -0800
From:   Sidhartha Kumar <sidhartha.kumar@...cle.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, songmuchun@...edance.com,
        tsahu@...ux.ibm.com, david@...hat.com
Subject: Re: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero
 support

On 12/8/22 2:01 PM, John Hubbard wrote:
> On 12/8/22 13:58, Sidhartha Kumar wrote:
>> Thanks John, Mike, Matthew, and Muchun for the feedback.
>>
>> To summarize this discussion and outline the next version of this 
>> patch, the changes I'll make include:
>>
>> 1) change the name of folio_set_compound_order() to folio_set_order()
>> 2) change the placement of this function from mm.h to mm/internal.h
>> 3) folio_set_order() will set both _folio_order and _folio_nr_pages 
>> and handle the zero order case correctly.
>> 4) remove the comment about hugetlb's specific use for zero orders
>> 5) improve the style of folio_set_order() by removing ifdefs from 
>> inside the function to doing
>>
>> #ifdef CONFIG_64BIT
>>   static inline void folio_set_order(struct folio *folio,
>>                   unsigned int order)
>>   {
>>       VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> 
> Sounds good, except for this part: why is a function named
> folio_set_order() BUG-ing on a non-large folio? The naming
> is still wrong, perhaps?
> 

This is because the _folio_nr_pages and _folio_order fields are part of 
the first tail page in the folio. folio_test_large returns if the folio 
is larger than one page which would be required for setting the fields.

>>
>>       folio->_folio_order = order;
>>           folio->_folio_nr_pages = order ? 1U << order : 0;
>> }
>> #else
>> static inline void folio_set_order(struct folio *folio,
>>                   unsigned int order)
>>   {
>>       VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
>>
>>       folio->_folio_order = order;
>> }
>> #endif
>>
>> Please let me know if I missing something.
>> Thanks,
>> Sidhartha Kumar
>>> Thanks,
>>
> 
> thanks,