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

[John Hubbard]

On 12/7/22 14:37, Sidhartha Kumar wrote:
> Document hugetlb's use of a zero compound order so support for zero
> orders is not removed from folio_set_compound_order().
>
> Signed-off-by: Sidhartha Kumar <sidhartha.kumar@xxxxxxxxxx>
> Suggested-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
> Suggested-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
> ---
> This can be folded into f2b67a51d0ef6871d4fb0c3e8199f278112bd108
> mm: add folio dtor and order setter functions
>
>   include/linux/mm.h | 7 +++++++
>   1 file changed, 7 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 443d496949a8..cd8508d728f1 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -999,9 +999,16 @@ static inline void set_compound_order(struct page *page, unsigned int order)
>   #endif
>   }
>   
> +/*
> + * folio_set_compound_order is generally passed a non-zero order to
> + * initialize a large folio.  However, hugetlb code abuses this by
> + * passing in zero when 'dissolving' a large folio.
> + */

Wouldn't it be better to instead just create a new function for that
case, such as:

    dissolve_large_folio()

?

>   static inline void folio_set_compound_order(struct folio *folio,
>   		unsigned int order)
>   {
> +	VM_BUG_ON_FOLIO(!folio_test_large(folio), folio);
> +
>   	folio->_folio_order = order;
>   #ifdef CONFIG_64BIT
>   	folio->_folio_nr_pages = order ? 1U << order : 0;

thanks,
--
John Hubbard
NVIDIA