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

["Pankaj Raghav (Samsung)" <kernel@pankajraghav.com>]

From: Pankaj Raghav <p.raghav@samsung.com>

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@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 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