[PATCH v2 09/14] mm/page-flags: check page/folio->private instead of PG_private

@@ -232,7 +232,6 @@ u64 stable_page_flags(const struct page *page)
 
 	u |= kpf_copy_bit(k, KPF_RESERVED,	PG_reserved);
 	u |= kpf_copy_bit(k, KPF_OWNER_2,	PG_owner_2);
-	u |= kpf_copy_bit(k, KPF_PRIVATE,	PG_private);
 	u |= kpf_copy_bit(k, KPF_PRIVATE_2,	PG_private_2);
 	u |= kpf_copy_bit(k, KPF_OWNER_PRIVATE,	PG_owner_priv_1);
 	u |= kpf_copy_bit(k, KPF_ARCH,		PG_arch_1);

@@ -11,7 +11,6 @@
 #define KPF_RESERVED		32
 #define KPF_MLOCKED		33
 #define KPF_OWNER_2		34
-#define KPF_PRIVATE		35
 #define KPF_PRIVATE_2		36
 #define KPF_OWNER_PRIVATE	37
 #define KPF_ARCH		38

@@ -3022,9 +3022,9 @@ static inline bool folio_maybe_mapped_shared(struct folio *folio)
  * @folio: the folio
  *
  * Calculate the expected folio refcount, taking references from the pagecache,
- * swapcache, PG_private and page table mappings into account. Useful in
- * combination with folio_ref_count() to detect unexpected references (e.g.,
- * GUP or other temporary references).
+ * swapcache, private data (folio->private != NULL) and page table mappings into
+ * account. Useful in combination with folio_ref_count() to detect unexpected
+ * references (e.g., GUP or other temporary references).
  *
  * Does currently not consider references from the LRU cache. If the folio
  * was isolated from the LRU (which is the case during migration or split),

@@ -3062,10 +3062,18 @@ static inline int folio_expected_ref_count(const struct folio *folio)
 	ref_count += folio_test_swapcache(folio) << order;
 
 	if (!folio_test_anon(folio)) {
-		/* One reference per page from the pagecache. */
-		ref_count += !!folio->mapping << order;
-		/* One reference from PG_private. */
-		ref_count += folio_test_private(folio);
+		/*
+		 * One reference per page from the pagecache.
+		 * Use data_race() since folio might not be locked.
+		 */
+		ref_count += !!data_race(folio->mapping) << order;
+		/*
+		 * One reference from filesystem private data.
+		 * Use data_race() since folio might not be locked.
+		 */
+		ref_count += data_race(folio_test_private(folio)) &&
+			     !folio_test_hugetlb(folio) &&
+			     !folio_test_swapcache(folio);
 	}
 
 	/* One reference per page table mapping. */

@@ -578,7 +578,23 @@ FOLIO_FLAG(swapbacked, FOLIO_HEAD_PAGE)
  * for its own purposes.
  * - PG_private and PG_private_2 cause release_folio() and co to be invoked
  */
-PAGEFLAG(Private, private, PF_ANY)
+
+static __always_inline bool folio_test_private(const struct folio *folio)
+{
+	return folio->private;
+}
+
+static __always_inline int PagePrivate(const struct page *page)
+{
+	return !!page->private;
+}
+
+/* no-ops during transition */
+static __always_inline void folio_set_private(struct folio *folio) { }
+static __always_inline void folio_clear_private(struct folio *folio) { }
+static __always_inline void SetPagePrivate(struct page *page) { }
+static __always_inline void ClearPagePrivate(struct page *page) { }
+
 FOLIO_FLAG(private_2, FOLIO_HEAD_PAGE)
 
 /* owner_2 can be set on tail pages for anon memory */

@@ -1170,7 +1186,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  */
 #define PAGE_FLAGS_CHECK_AT_FREE				\
 	(1UL << PG_lru		| 1UL << PG_locked	|	\
-	 1UL << PG_private	| 1UL << PG_private_2	|	\
+	 1UL << PG_private_2	|				\
 	 1UL << PG_writeback	| 1UL << PG_reserved	|	\
 	 1UL << PG_active 	|				\
 	 1UL << PG_unevictable	| __PG_MLOCKED | LRU_GEN_MASK)

@@ -1194,8 +1210,6 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
 	(0xffUL /* order */		| 1UL << PG_has_hwpoisoned |	\
 	 1UL << PG_large_rmappable	| 1UL << PG_partially_mapped)
 
-#define PAGE_FLAGS_PRIVATE				\
-	(1UL << PG_private | 1UL << PG_private_2)
 /**
  * folio_has_private - Determine if folio has private stuff
  * @folio: The folio to be checked

@@ -1205,7 +1219,9 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  */
 static inline int folio_has_private(const struct folio *folio)
 {
-	return !!(folio->flags.f & PAGE_FLAGS_PRIVATE);
+	return (!!folio->private && !folio_test_swapcache(folio) &&
+		!folio_test_hugetlb(folio)) ||
+		folio_test_private_2(folio);
 }
 
 #undef PF_ANY

@@ -22,7 +22,9 @@
 	(folio_test_swapcache(folio)	? PAGEMAP_SWAPCACHE  : 0) | \
 	(folio_test_swapbacked(folio)	? PAGEMAP_SWAPBACKED : 0) | \
 	(folio_test_mappedtodisk(folio)	? PAGEMAP_MAPPEDDISK : 0) | \
-	(folio_test_private(folio)	? PAGEMAP_BUFFERS    : 0) \
+	(folio_test_private(folio) && \
+	 !folio_test_swapcache(folio) && \
+	 !folio_test_hugetlb(folio)	? PAGEMAP_BUFFERS    : 0) \
 	)
 
 TRACE_EVENT(mm_lru_insertion,

@@ -4810,7 +4810,9 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 		 * will try to drop it before split and then check if the folio
 		 * can be split or not. So skip the check here.
 		 */
-		if (!folio_test_private(folio) &&
+		if (!(folio_test_private(folio) &&
+		      !folio_test_swapcache(folio) &&
+		      !folio_test_hugetlb(folio)) &&
 		    folio_expected_ref_count(folio) != folio_ref_count(folio))
 			goto next;
 

@@ -1327,7 +1327,8 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 	 * free the metadata, so the page can be freed.
 	 */
 	if (!src->mapping) {
-		if (folio_test_private(src)) {
+		if (folio_test_private(src) && !folio_test_swapcache(src) &&
+		    !folio_test_hugetlb(src)) {
 			try_to_free_buffers(src);
 			goto out;
 		}

@@ -2705,7 +2705,10 @@ bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
 	if (folio_test_set_dirty(folio))
 		return false;
 
-	__folio_mark_dirty(folio, mapping, !folio_test_private(folio));
+	__folio_mark_dirty(folio, mapping,
+			   !(folio_test_private(folio) &&
+			     !folio_test_swapcache(folio) &&
+			     !folio_test_hugetlb(folio)));
 
 	if (mapping->host) {
 		/* !PageAnon && !swapper_space */

@@ -955,7 +955,8 @@ static void folio_check_dirty_writeback(struct folio *folio,
 	*writeback = folio_test_writeback(folio);
 
 	/* Verify dirty/writeback state if the filesystem supports it */
-	if (!folio_test_private(folio))
+	if (!(folio_test_private(folio) && !folio_test_swapcache(folio) &&
+	      !folio_test_hugetlb(folio)))
 		return;
 
 	mapping = folio_mapping(folio);

@@ -73,7 +73,6 @@
 #define KPF_RESERVED		32
 #define KPF_MLOCKED		33
 #define KPF_OWNER_2		34
-#define KPF_PRIVATE		35
 #define KPF_PRIVATE_2		36
 #define KPF_OWNER_PRIVATE	37
 #define KPF_ARCH		38

@@ -131,7 +130,6 @@ static const char * const page_flag_names[] = {
 	[KPF_RESERVED]		= "r:reserved",
 	[KPF_MLOCKED]		= "m:mlocked",
 	[KPF_OWNER_2]		= "d:owner_2",
-	[KPF_PRIVATE]		= "P:private",
 	[KPF_PRIVATE_2]		= "p:private_2",
 	[KPF_OWNER_PRIVATE]	= "O:owner_private",
 	[KPF_ARCH]		= "h:arch",