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

[Dave Hansen]

On 6/12/25 03:50, Pankaj Raghav wrote:
> +/*
> + * mm_get_static_huge_zero_folio - Get a PMD sized zero folio

Isn't that a rather inaccurate function name and comment?

The third line of the function literally returns a non-PMD-sized zero folio.

> + * 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 *mm_get_static_huge_zero_folio(void)
> +{
> +	if(IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE))
> +		return READ_ONCE(huge_zero_folio);
> +	return page_folio(ZERO_PAGE(0));
> +}

This doesn't tell us very much about when I should use:

	mm_get_static_huge_zero_folio()
vs.
	mm_get_huge_zero_folio(mm)
vs.
	page_folio(ZERO_PAGE(0))

What's with the "mm_" in the name? Usually "mm" means "mm_struct" not
Memory Management. It's really weird to prefix something that doesn't
take an "mm_struct" with "mm_"

Isn't the "get_" also a bad idea since mm_get_huge_zero_folio() does its
own refcounting but this interface does not?

Shouldn't this be something more along the lines of:

/*
 * pick_zero_folio() - Pick and return the largest available zero folio
 *
 * mm_get_huge_zero_folio() is preferred over this function. It is more
 * flexible and can provide a larger zero page under wider
 * circumstances.
 *
 * Only use this when there is no mm available.
 *
 * ... then other comments
 */
static inline struct folio *pick_zero_folio(void)
{
	if (IS_ENABLED(CONFIG_STATIC_PMD_ZERO_PAGE))
		return READ_ONCE(huge_zero_folio);
	return page_folio(ZERO_PAGE(0));
}

Or, maybe even name it _just_: zero_folio()