Re: [PATCH v2 4/5] mm: add largest_zero_folio() routine

[David Hildenbrand <david@redhat.com>]

On 07.07.25 16:23, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@samsung.com>
> 
> Add largest_zero_folio() routine so that huge_zero_folio can be
> used without the need to pass any mm struct. This will return ZERO_PAGE
> folio if CONFIG_STATIC_PMD_ZERO_PAGE is disabled or if we failed to
> allocate a PMD page from memblock.
> 
> This routine can also be called even if THP is disabled.
> 
> Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
> ---
>   include/linux/mm.h | 28 ++++++++++++++++++++++++++--
>   1 file changed, 26 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 428fe6d36b3c..d5543cf7b8e9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4018,17 +4018,41 @@ static inline bool vma_is_special_huge(const struct vm_area_struct *vma)
>   
>   #endif /* CONFIG_TRANSPARENT_HUGEPAGE || CONFIG_HUGETLBFS */
>   
> +extern struct folio *huge_zero_folio;
> +extern unsigned long huge_zero_pfn;

No need for "extern".

> +
>   #ifdef CONFIG_STATIC_PMD_ZERO_PAGE
>   extern void __init static_pmd_zero_init(void);
> +
> +/*
> + * largest_zero_folio - Get the largest zero size folio available
> + *
> + * This function will return a PMD sized zero folio if CONFIG_STATIC_PMD_ZERO_PAGE
> + * is enabled. Otherwise, a ZERO_PAGE folio is returned.
> + *
> + * Deduce the size of the folio with folio_size instead of assuming the
> + * folio size.
> + */
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	if(!huge_zero_folio)

Nit: if (!huge_zero_folio), but see below

I assume this check is only in place to handle whether static allocation 
failed, correct?

> +		return page_folio(ZERO_PAGE(0));
> +
> +	return READ_ONCE(huge_zero_folio);

READ_ONCE should not be required if it cannot get freed.

> +}
> +
>   #else
>   static inline void __init static_pmd_zero_init(void)
>   {
>   	return;
>   }
> +
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	return page_folio(ZERO_PAGE(0));
> +}
>   #endif

Could we do:

static inline struct folio *largest_zero_folio(void)
{
	if (IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE) &&
  	    likely(huge_zero_folio))
		return huge_zero_folio;
	return page_folio(ZERO_PAGE(0));
}
	

-- 
Cheers,

David / dhildenb