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Date:   Thu, 8 Dec 2022 14:12:18 -0800
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Subject: Re: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero
 support
Content-Language: en-US
To:     John Hubbard <jhubbard@nvidia.com>,
        Mike Kravetz <mike.kravetz@oracle.com>,
        Matthew Wilcox <willy@infradead.org>
Cc:     linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        akpm@linux-foundation.org, songmuchun@bytedance.com,
        tsahu@linux.ibm.com, david@redhat.com
References: <20221207223731.32784-1-sidhartha.kumar@oracle.com>
 <92965844-c430-8b8e-d9f1-705d7578bceb@nvidia.com>
 <ec8f46ca-9ea6-4567-2038-22f6d3000ed5@oracle.com>
 <d17530ad-8e12-8069-d619-a2d72fe80e15@nvidia.com>
 <0187f9c2-e80a-9cde-68bc-c9bdbd96b6fe@oracle.com>
 <Y5I78soNmAFv7pi8@casper.infradead.org> <Y5JCi3h8bUzLf3cu@monkey>
 <2723541a-79aa-c6b5-d82c-53db76b78145@oracle.com>
 <e86ca90f-e59e-3851-7225-b5f596ad04b9@nvidia.com>
From:   Sidhartha Kumar <sidhartha.kumar@oracle.com>
In-Reply-To: <e86ca90f-e59e-3851-7225-b5f596ad04b9@nvidia.com>
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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,