On 12.06.25 22:54, Pankaj Raghav (Samsung) wrote:

On Thu, Jun 12, 2025 at 07:09:34AM -0700, Dave Hansen wrote:

quoted

On 6/12/25 03:50, Pankaj Raghav wrote:

quoted

+/*
+ * mm_get_static_huge_zero_folio - Get a PMD sized zero folio

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

I agree. I also felt it was not a good name for the function.

quoted

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

quoted

+ * 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_"

Got it. Actually, I was not aware of this one.

quoted

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

Agree.

quoted

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()

I think zero_folio() sounds like a good and straightforward name. In
most cases it will return a ZERO_PAGE() folio. If
CONFIG_STATIC_PMD_ZERO_PAGE is enabled, then we return a PMD page.

"zero_folio" would be confusing I'm afraid.

At least with current "is_zero_folio" etc.

"largest_zero_folio" or sth. like that might make it clearer that the 
size we are getting back might actually differ.

-- 
Cheers,

David / dhildenb