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

[Matthew Wilcox <willy@xxxxxxxxxxxxx>]

On Thu, Dec 08, 2022 at 01:58:20PM -0800, Sidhartha Kumar wrote:
> 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);
> 
> 	 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

While we usually prefer to put ifdefs outside the function, I don't
think that's justified in this case.  I'd rather see a comment inside
the function like:

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;
#ifdef CONFIG_64BIT
	/*
	 * When hugetlb dissolves a folio, we need to clear the tail
	 * page, rather than setting nr_pages to 1.
	 */
	folio->_folio_nr_pages = order ? 1U << order : 0;
#endif
}