[PATCH v1] mm: remove total_mapcount()

[David Hildenbrand]

mm/memfd.c is that last remaining user of total_mapcount().  Let's
convert memfd_tag_pins() and memfd_wait_for_pins() to use folios
instead of pages, so we can remove total_mapcount() for good.

We always get a head page, so we can just naturally interpret is as a folio
(similar to other code).

Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Matthew Wilcox (Oracle) <willy@xxxxxxxxxxxxx>
Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
---

Did a quick test with write-sealing a memfd backed by THP. Seems to work
as it used to.

---
 include/linux/mm.h |  9 +--------
 mm/memfd.c         | 34 ++++++++++++++++++----------------
 2 files changed, 19 insertions(+), 24 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 6f4825d82965..49e22a2f6ccc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1183,7 +1183,7 @@ static inline int is_vmalloc_or_module_addr(const void *x)
  * How many times the entire folio is mapped as a single unit (eg by a
  * PMD or PUD entry).  This is probably not what you want, except for
  * debugging purposes - it does not include PTE-mapped sub-pages; look
- * at folio_mapcount() or page_mapcount() or total_mapcount() instead.
+ * at folio_mapcount() or page_mapcount() instead.
  */
 static inline int folio_entire_mapcount(struct folio *folio)
 {
@@ -1243,13 +1243,6 @@ static inline int folio_mapcount(struct folio *folio)
 	return folio_total_mapcount(folio);
 }
 
-static inline int total_mapcount(struct page *page)
-{
-	if (likely(!PageCompound(page)))
-		return atomic_read(&page->_mapcount) + 1;
-	return folio_total_mapcount(page_folio(page));
-}
-
 static inline bool folio_large_is_mapped(struct folio *folio)
 {
 	/*
diff --git a/mm/memfd.c b/mm/memfd.c
index d3a1ba4208c9..0a6c1a6ee03b 100644
--- a/mm/memfd.c
+++ b/mm/memfd.c
@@ -31,24 +31,25 @@
 
 static void memfd_tag_pins(struct xa_state *xas)
 {
-	struct page *page;
+	struct folio *folio;
 	int latency = 0;
 	int cache_count;
 
 	lru_add_drain();
 
 	xas_lock_irq(xas);
-	xas_for_each(xas, page, ULONG_MAX) {
+	xas_for_each(xas, folio, ULONG_MAX) {
 		cache_count = 1;
-		if (!xa_is_value(page) &&
-		    PageTransHuge(page) && !PageHuge(page))
+		if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+		    !xa_is_value(folio) && folio_test_large(folio) &&
+		    !folio_test_hugetlb(folio))
 			cache_count = HPAGE_PMD_NR;
 
-		if (!xa_is_value(page) &&
-		    page_count(page) - total_mapcount(page) != cache_count)
+		if (!xa_is_value(folio) && cache_count !=
+		    folio_ref_count(folio) - folio_mapcount(folio))
 			xas_set_mark(xas, MEMFD_TAG_PINNED);
 		if (cache_count != 1)
-			xas_set(xas, page->index + cache_count);
+			xas_set(xas, folio->index + cache_count);
 
 		latency += cache_count;
 		if (latency < XA_CHECK_SCHED)
@@ -66,16 +67,16 @@ static void memfd_tag_pins(struct xa_state *xas)
 /*
  * Setting SEAL_WRITE requires us to verify there's no pending writer. However,
  * via get_user_pages(), drivers might have some pending I/O without any active
- * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all pages
+ * user-space mappings (eg., direct-IO, AIO). Therefore, we look at all folios
  * and see whether it has an elevated ref-count. If so, we tag them and wait for
  * them to be dropped.
  * The caller must guarantee that no new user will acquire writable references
- * to those pages to avoid races.
+ * to those folios to avoid races.
  */
 static int memfd_wait_for_pins(struct address_space *mapping)
 {
 	XA_STATE(xas, &mapping->i_pages, 0);
-	struct page *page;
+	struct folio *folio;
 	int error, scan;
 
 	memfd_tag_pins(&xas);
@@ -95,20 +96,21 @@ static int memfd_wait_for_pins(struct address_space *mapping)
 
 		xas_set(&xas, 0);
 		xas_lock_irq(&xas);
-		xas_for_each_marked(&xas, page, ULONG_MAX, MEMFD_TAG_PINNED) {
+		xas_for_each_marked(&xas, folio, ULONG_MAX, MEMFD_TAG_PINNED) {
 			bool clear = true;
 
 			cache_count = 1;
-			if (!xa_is_value(page) &&
-			    PageTransHuge(page) && !PageHuge(page))
+			if (IS_ENABLED(CONFIG_TRANSPARENT_HUGEPAGE) &&
+			    !xa_is_value(folio) && folio_test_large(folio) &&
+			    !folio_test_hugetlb(folio))
 				cache_count = HPAGE_PMD_NR;
 
-			if (!xa_is_value(page) && cache_count !=
-			    page_count(page) - total_mapcount(page)) {
+			if (!xa_is_value(folio) && cache_count !=
+			    folio_ref_count(folio) - folio_mapcount(folio)) {
 				/*
 				 * On the last scan, we clean up all those tags
 				 * we inserted; but make a note that we still
-				 * found pages pinned.
+				 * found folios pinned.
 				 */
 				if (scan == LAST_SCAN)
 					error = -EBUSY;
-- 
2.43.2