Re: [PATCH mm-unstable] mm: clarify folio_set_compound_order() zero support

[Muchun Song <muchun.song@linux.dev>]

> On Dec 9, 2022, at 06:39, John Hubbard <jhubbard@nvidia.com> wrote:
> 
> 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.

From you advise, I think we can remove VM_BUG_ON and handle non-zero
order page, something like:

static inline void folio_set_order(struct folio *folio,
		                   unsigned int order)
{
	if (!folio_test_large(folio)) {
		WARN_ON(order);
		return;
	}

	folio->_folio_order = order;
#ifdef CONFIG_64BIT
	folio->_folio_nr_pages = order ? 1U << order : 0;
#endif
}

In this case,

  1) we can handle both non-zero and zero (folio_order() works as well
     for this case) order page.
  2) it can prevent OOB for non-large folio and warn unexpected users.
  3) Do not BUG.
  4) No need to rename folio_set_order.

What do you think?

Thanks.

> 
> 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