[PATCH v3 10/14] mm/page-flags: introduce folio_test_fs_private()

[Zi Yan]

folio_test_fs_private() wraps folio->private != NULL check and excludes
swapcache and hugetlb folios, since swapcache uses swp_entry_t overlapping
with folio->private and hugetlb sets its own flags in folio->private.
Replace open code with the helper, since core MM does this check
frequently.

No functional change intended.

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>
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: linux-kernel@xxxxxxxxxxxxxxx
Cc: linux-fsdevel@xxxxxxxxxxxxxxx
Cc: linux-mm@xxxxxxxxx
Cc: linux-trace-kernel@xxxxxxxxxxxxxxx
---
 include/linux/mm.h             |  4 +---
 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 +--
 7 files changed, 29 insertions(+), 20 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5eb8a62fafb56..c861525cda83f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3053,9 +3053,7 @@ static inline int folio_expected_ref_count(const struct folio *folio)
 		 * 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_swapbacked(folio);
+		ref_count += data_race(folio_test_fs_private(folio));
 	}
 
 	/* One reference per page table mapping. */
diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
index 9cb4d64739798..ce7fccd90367b 100644
--- a/include/linux/page-flags.h
+++ b/include/linux/page-flags.h
@@ -1210,6 +1210,28 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
 	(0xffUL /* order */		| 1UL << PG_has_hwpoisoned |	\
 	 1UL << PG_large_rmappable	| 1UL << PG_partially_mapped)
 
+/**
+ * folio_test_fs_private - check if the folio has filesystem private data
+ * @folio: The folio to check.
+ *
+ * Use this in code that may encounter swapcache or hugetlb folios but only
+ * wants to detect filesystem private data. Swapcache stores swp_entry_t in
+ * folio->swap, a union with folio->private, and hugetlb stores its own flags
+ * in folio->private; both are excluded.
+ *
+ * NOTE: For swapcache, folio->swap.val PG_swapcache are not set as a whole,
+ * so folio_test_swapcache() is not reliable to exclude swapcache.
+ * Use folio_test_swapbacked() instead, since it remains set when a folio is
+ * added to/removed from swapcache.
+ *
+ * Return: true if folio->private is set and the folio is neither swapcache
+ * nor hugetlb.
+ */
+static inline bool folio_test_fs_private(const struct folio *folio)
+{
+	return folio_test_private(folio) && !folio_test_swapbacked(folio) &&
+	       !folio_test_hugetlb(folio);
+}
 /**
  * folio_has_private - Determine if folio has private stuff
  * @folio: The folio to be checked
@@ -1219,9 +1241,7 @@ static __always_inline void __ClearPageAnonExclusive(struct page *page)
  */
 static inline int folio_has_private(const struct folio *folio)
 {
-	return (!!folio->private && !folio_test_swapbacked(folio) &&
-		!folio_test_hugetlb(folio)) ||
-		folio_test_private_2(folio);
+	return folio_test_fs_private(folio) || folio_test_private_2(folio);
 }
 
 #undef PF_ANY
diff --git a/include/trace/events/pagemap.h b/include/trace/events/pagemap.h
index 8193f91216822..547a550b81ee1 100644
--- a/include/trace/events/pagemap.h
+++ b/include/trace/events/pagemap.h
@@ -23,9 +23,7 @@
 	(folio_test_swapbacked(folio)	? PAGEMAP_SWAPBACKED : 0) | \
 	(folio_test_mappedtodisk(folio)	? PAGEMAP_MAPPEDDISK : 0) | \
 	/* data_race() is used to read folio->private locklessly */ \
-	(data_race(folio_test_private(folio)) && \
-	 !folio_test_swapbacked(folio) && \
-	 !folio_test_hugetlb(folio)	? PAGEMAP_BUFFERS    : 0) \
+	(data_race(folio_test_fs_private(folio))	? PAGEMAP_BUFFERS    : 0) \
 	)
 
 TRACE_EVENT(mm_lru_insertion,
diff --git a/mm/huge_memory.c b/mm/huge_memory.c
index 9b2a9d0794a61..4cb7d9bc47544 100644
--- a/mm/huge_memory.c
+++ b/mm/huge_memory.c
@@ -4833,9 +4833,7 @@ static int split_huge_pages_pid(int pid, unsigned long vaddr_start,
 		 * can be split or not. So skip the check here.
 		 * data_race() is used to read folio->private locklessly.
 		 */
-		if (!(data_race(folio_test_private(folio)) &&
-		      !folio_test_swapbacked(folio) &&
-		      !folio_test_hugetlb(folio)) &&
+		if (!data_race(folio_test_fs_private(folio)) &&
 		    folio_expected_ref_count(folio) != folio_ref_count(folio))
 			goto next;
 
diff --git a/mm/migrate.c b/mm/migrate.c
index f6befd3ff1c46..2e133d533c8da 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1327,8 +1327,7 @@ 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) && !folio_test_swapbacked(src) &&
-		    !folio_test_hugetlb(src)) {
+		if (folio_test_fs_private(src)) {
 			try_to_free_buffers(src);
 			goto out;
 		}
diff --git a/mm/page-writeback.c b/mm/page-writeback.c
index 02ad49b10be07..e7389020d2a56 100644
--- a/mm/page-writeback.c
+++ b/mm/page-writeback.c
@@ -2706,10 +2706,7 @@ bool filemap_dirty_folio(struct address_space *mapping, struct folio *folio)
 		return false;
 
 	/* data_race() is used to read folio->private locklessly */
-	__folio_mark_dirty(folio, mapping,
-			   !(data_race(folio_test_private(folio)) &&
-			     !folio_test_swapbacked(folio) &&
-			     !folio_test_hugetlb(folio)));
+	__folio_mark_dirty(folio, mapping, !data_race(folio_test_fs_private(folio)));
 
 	if (mapping->host) {
 		/* !PageAnon && !swapper_space */
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 9348ebf9de882..47352bafc5b6f 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -978,8 +978,7 @@ 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) && !folio_test_swapbacked(folio) &&
-	      !folio_test_hugetlb(folio)))
+	if (!folio_test_fs_private(folio))
 		return;
 
 	mapping = folio_mapping(folio);

-- 
2.53.0