[RFC PATCH v4 01/14] mm: Consolidate freeing of typed folios on final folio_put()

[Fuad Tabba <tabba@google.com>]

Some folio types, such as hugetlb, handle freeing their own
folios. Moreover, guest_memfd will require being notified once a
folio's reference count reaches 0 to facilitate shared to private
folio conversion, without the folio actually being freed at that
point.

As a first step towards that, this patch consolidates freeing
folios that have a type. The first user is hugetlb folios. Later
in this patch series, guest_memfd will become the second user of
this.

Suggested-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Fuad Tabba <tabba@google.com>
---
 include/linux/page-flags.h | 15 +++++++++++++++
 mm/swap.c                  | 24 +++++++++++++++++++-----
 2 files changed, 34 insertions(+), 5 deletions(-)

diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index cf46ac720802..aca57802d7c7 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -970,6 +970,21 @@ static inline bool page_has_type(const struct page *page)
 	return page_mapcount_is_type(data_race(page->page_type));
 }
 
+static inline int page_get_type(const struct page *page)
+{
+	return page->page_type >> 24;
+}
+
+static inline bool folio_has_type(const struct folio *folio)
+{
+	return page_has_type(&folio->page);
+}
+
+static inline int folio_get_type(const struct folio *folio)
+{
+	return page_get_type(&folio->page);
+}
+
 #define FOLIO_TYPE_OPS(lname, fname)					\
 static __always_inline bool folio_test_##fname(const struct folio *folio) \
 {									\
diff --git a/mm/swap.c b/mm/swap.c
index 10decd9dffa1..6f01b56bce13 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -94,6 +94,20 @@ static void page_cache_release(struct folio *folio)
 		unlock_page_lruvec_irqrestore(lruvec, flags);
 }
 
+static void free_typed_folio(struct folio *folio)
+{
+	switch (folio_get_type(folio)) {
+	case PGTY_hugetlb:
+		free_huge_folio(folio);
+		return;
+	case PGTY_offline:
+		/* Nothing to do, it's offline. */
+		return;
+	default:
+		WARN_ON_ONCE(1);
+	}
+}
+
 void __folio_put(struct folio *folio)
 {
 	if (unlikely(folio_is_zone_device(folio))) {
@@ -101,8 +115,8 @@ void __folio_put(struct folio *folio)
 		return;
 	}
 
-	if (folio_test_hugetlb(folio)) {
-		free_huge_folio(folio);
+	if (unlikely(folio_has_type(folio))) {
+		free_typed_folio(folio);
 		return;
 	}
 
@@ -934,13 +948,13 @@ void folios_put_refs(struct folio_batch *folios, unsigned int *refs)
 		if (!folio_ref_sub_and_test(folio, nr_refs))
 			continue;
 
-		/* hugetlb has its own memcg */
-		if (folio_test_hugetlb(folio)) {
+		if (unlikely(folio_has_type(folio))) {
+			/* typed folios have their own memcg, if any */
 			if (lruvec) {
 				unlock_page_lruvec_irqrestore(lruvec, flags);
 				lruvec = NULL;
 			}
-			free_huge_folio(folio);
+			free_typed_folio(folio);
 			continue;
 		}
 		folio_unqueue_deferred_split(folio);
-- 
2.47.1.613.gc27f4b7a9f-goog