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

[John Hubbard <jhubbard@nvidia.com>]

On 12/9/22 13:10, John Hubbard wrote:
> On 12/9/22 06:27, Muchun Song wrote:
>>  From you advise, I think we can remove VM_BUG_ON and handle non-zero
>> order page, something like:
> 
> Yes, and thanks for summarizing all the individual feedback into a
> proposed solution.
> 
> If we go this route, then I'd suggest a little note above the function,
> such as:
> 
> /*
>   * For non-large folios, this will have no effect, other than possibly
>   * generating a warning, if the caller attempts to set a non-zero folio order
>   * for a non-large folio.
>   */
> 
>> static inline void folio_set_order(struct folio *folio,
>>                            unsigned int order)
>> {
>>     if (!folio_test_large(folio)) {
>>         WARN_ON(order);

Although, on second thought...I'm still a little confused about why
keeping the same name is so important?

A very direct approach that has more accurate naming (and therefore no
need for a strange comment explaining the behavior) would be:


static inline void large_folio_set_order(struct folio *folio,
					 unsigned int order)
{
	if (WARN_ON_ONCE(!folio_test_large(folio)))
		return;

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


thanks,
-- 
John Hubbard
NVIDIA