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

[Zi Yan]

After the changes of the prior commits, page/folio->private != NULL is now
equivalent to checking PG_private.

Stop checking PG_private on pages and folios and use page/folio->private
instead, except swapcache and hugetlb folios, because the former uses a
field (swp_entry_t swap) overlapping with ->private and the latter sets its
flags in ->private. Exclude swapcache and hugetlb when the code is meant to
check PG_private only.

folio_set/clear_private() and Set/ClearPagePrivate() become no-ops.
PG_private is no longer checked at page free time.

KPF_PRIVATE exposes PG_private to userspace. Change its code logic to check
folio->private != NULL and exclude non-pagecache, swapcache, hugetlb, and
anon folios. One minor semantic change, for orphaned pagecache folios
(mapping == NULL) with fs-private data will no longer have KPF_PRIVATE.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
To: David Hildenbrand <david@xxxxxxxxxx>
To: Steven Rostedt <rostedt@xxxxxxxxxxx>
To: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
To: Lorenzo Stoakes <ljs@xxxxxxxxxx>
To: "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx>
To: Jan Kara <jack@xxxxxxx>
To: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: "Liam R. Howlett" <liam@xxxxxxxxxxxxx>
Cc: Vlastimil Babka <vbabka@xxxxxxxxxx>
Cc: Mike Rapoport <rppt@xxxxxxxxxx>
Cc: Suren Baghdasaryan <surenb@xxxxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Zi Yan <ziy@xxxxxxxxxx>
Cc: Baolin Wang <baolin.wang@xxxxxxxxxxxxxxxxx>
Cc: Nico Pache <nico.pache@xxxxxxxxx>
Cc: Ryan Roberts <ryan.roberts@xxxxxxx>
Cc: Dev Jain <dev.jain@xxxxxxx>
Cc: Barry Song <baohua@xxxxxxxxxx>
Cc: Lance Yang <lance.yang@xxxxxxxxx>
Cc: Usama Arif <usama.arif@xxxxxxxxx>
Cc: Matthew Brost <matthew.brost@xxxxxxxxx>
Cc: Joshua Hahn <joshua.hahnjy@xxxxxxxxx>
Cc: Rakie Kim <rakie.kim@xxxxxx>
Cc: Byungchul Park <byungchul@xxxxxx>
Cc: Gregory Price <gourry@xxxxxxxxxx>
Cc: Ying Huang <ying.huang@xxxxxxxxxxxxxxxxx>
Cc: Alistair Popple <apopple@xxxxxxxxxx>
Cc: Qi Zheng <qi.zheng@xxxxxxxxx>
Cc: Shakeel Butt <shakeel.butt@xxxxxxxxx>
Cc: Kairui Song <kasong@xxxxxxxxxxx>
Cc: Axel Rasmussen <axelrasmussen@xxxxxxxxxx>
Cc: Yuanchu Xie <yuanchu@xxxxxxxxxx>
Cc: Wei Xu <weixugc@xxxxxxxxxx>
Cc: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
---
 fs/proc/page.c                 |  6 +++++-
 include/linux/mm.h             | 11 ++++++-----
 include/linux/page-flags.h     | 26 +++++++++++++++++++++-----
 include/trace/events/pagemap.h |  4 +++-
 mm/huge_memory.c               |  4 +++-
 mm/migrate.c                   |  3 ++-
 mm/page-writeback.c            |  5 ++++-
 mm/vmscan.c                    |  3 ++-
 8 files changed, 46 insertions(+), 16 deletions(-)

diff --git a/fs/proc/page.c b/fs/proc/page.c
index 260772b20bd99..abfa6f7d890cc 100644
--- a/fs/proc/page.c
+++ b/fs/proc/page.c
@@ -232,7 +232,11 @@ 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);
+	/* preserve the original KPF_PRIVATE semantics by excluding non pagecache folios */
+	if (folio->mapping && !folio_test_anon(folio) &&
+	    (folio_get_private(folio) && !folio_test_swapcache(folio) &&
+	     !folio_test_hugetlb(folio)))
+		u |= BIT_ULL(KPF_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);
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7fabe6c66b4b7..ebc035ac26ccc 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2961,9 +2961,9 @@ 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),
@@ -3003,8 +3003,9 @@ static inline int folio_expected_ref_count(const struct folio *folio)
 	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 from filesystem private data. */
+		ref_count += !!folio->private && !folio_test_hugetlb(folio) &&
+			     !folio_test_swapcache(folio);
 	}
 
 	/* One reference per page table mapping. */
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 7a863572adce7..8efc61f967302 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -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(page);
+}
+
+/* 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
diff --git a/include/trace/events/pagemap.h b/include/trace/events/pagemap.h
index 36c3a90f0acca..fb9abec40ec79 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -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,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 21c92ee48e469..d21a9b8f40d15 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4799,7 +4799,9 @@ 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;
 
diff --git a/mm/migrate.c b/mm/migrate.c
index b937cbd764808..ad5daef8721cf 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1330,7 +1330,8 @@ 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;
 		}
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 47495be68598f..b44468383f900 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2706,7 +2706,10 @@ 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 */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 17d2b793cbfc4..ab059a2c6ea35 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -954,7 +954,8 @@ 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);

-- 
2.53.0