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

[Lorenzo Stoakes]

On Fri, Aug 08, 2025 at 02:11:40PM +0200, Pankaj Raghav (Samsung) wrote:
> From: Pankaj Raghav <p.raghav@xxxxxxxxxxx>
>
> The callers of mm_get_huge_zero_folio() have access to a mm struct and
> the lifetime of the huge_zero_folio is tied to the lifetime of the mm
> struct.
>
> largest_zero_folio() will give access to huge_zero_folio when
> PERSISTENT_HUGE_ZERO_FOLIO config option is enabled for callers that do not
> want to tie the lifetime to a mm struct. This is very useful for
> filesystem and block layers where the request completions can be async
> and there is no guarantee on the mm struct lifetime.
>
> This function will return a ZERO_PAGE folio if PERSISTENT_HUGE_ZERO_FOLIO
> is disabled or if we failed to allocate a huge_zero_folio during early
> init.
>
> Co-developed-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Pankaj Raghav <p.raghav@xxxxxxxxxxx>

Hm thought I R-b this already :P

LGTM, so:

Reviewed-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>

> ---
>  include/linux/huge_mm.h | 22 ++++++++++++++++++++++
>  1 file changed, 22 insertions(+)
>
> diff --git a/include/linux/huge_mm.h b/include/linux/huge_mm.h
> index bd547857c6c1..14d424830fa8 100644
> --- a/include/linux/huge_mm.h
> +++ b/include/linux/huge_mm.h
> @@ -714,4 +714,26 @@ static inline int split_folio_to_order(struct folio *folio, int new_order)
>  	return split_folio_to_list_to_order(folio, NULL, new_order);
>  }
>
> +/**
> + * largest_zero_folio - Get the largest zero size folio available
> + *
> + * This function shall be used when mm_get_huge_zero_folio() cannot be
> + * used as there is no appropriate mm lifetime to tie the huge zero folio
> + * from the caller.
> + *
> + * Deduce the size of the folio with folio_size instead of assuming the
> + * folio size.
> + *
> + * Return: pointer to PMD sized zero folio if CONFIG_PERSISTENT_HUGE_ZERO_FOLIO
> + * is enabled or a single page sized zero folio
> + */
> +static inline struct folio *largest_zero_folio(void)
> +{
> +	struct folio *folio = get_persistent_huge_zero_folio();
> +
> +	if (folio)
> +		return folio;
> +
> +	return page_folio(ZERO_PAGE(0));
> +}
>  #endif /* _LINUX_HUGE_MM_H */
> --
> 2.49.0
>