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

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Date:   Thu, 8 Dec 2022 14:33:01 -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 2:14 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?


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