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

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Date:   Thu, 8 Dec 2022 14:39:45 -0800
From:   John Hubbard <jhubbard@...dia.com>
To:     Sidhartha Kumar <sidhartha.kumar@...cle.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 14:33, Sidhartha Kumar wrote:
> On 12/8/22 2:14 PM, John Hubbard wrote:
>> On 12/8/22 14:12, Sidhartha Kumar wrote:
>>> 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.
>>
>> OK, but then as I said, the name is wrong. One can either:
>>
>> a) handle the non-large case, or
>>
>> b) rename the function to indicate that it only works on large folios.
>>
> 
> Discussed here[1], the BUG_ON line seemed more appropriate over
> 
> if (!folio_test_large(folio))
>      return;
> 
> as the misuse would not be silent. I think I would be against renaming the function as I don't see any large folio specific function names for other accessors of tail page fields. Would both the BUG_ON and return on non-large folio be included then?

Actually, if you want the "misuse to not be silent", today's guidelines
would point more toward WARN and return, instead of BUG, btw.

I don't think that a survey of existing names is really the final word on what
to name this. Names should be accurate, even if other names are less so. How
about something like:

     large_folio_set_order()

?

> 
> 
> [1]: https://lore.kernel.org/linux-mm/20221129225039.82257-1-sidhartha.kumar@oracle.com/T/#m98cf80bb21ae533b7385f2e363c602e2c9e2802d
>>
>> thanks,
> 
> 

thanks,
-- 
John Hubbard
NVIDIA