[PATCH] mm/zsmalloc: replace PG_private with pointer comparison

[Zi Yan]

zsmalloc uses PG_private to indicate first zpdesc in a zspage chain. It is
equivalent to check zpdesc =3D=3D zspage->first_zpdesc. Replace
is_first_zpdesc() with zpdesc =3D=3D zspage->first_zpdesc in obj_allocated(=
).

For get_first_zpdesc(), first_zpdesc is from zspage->first_zpdesc, so
replace is_first_zpdesc() with first_zpdesc->zspage =3D=3D zspage, the seco=
nd
requirement of a zspage chain, where all zpdescs point to the same zspage.

is_first_zpdesc(), is only used in VM_BUG_ON_PAGE(), so performance impact
should be negligible. While at it, change VM_BUG_ON() to
VM_WARN_ON_ONCE_PAGE().

It prepares for a future commit that remove PG_private.

No functional change intended.

Assisted-by: Claude:claude-opus-4-8
Assisted-by: Codex:gpt-5
Signed-off-by: Zi Yan <ziy@xxxxxxxxxx>
To: Minchan Kim <minchan@xxxxxxxxxx>
To: Sergey Senozhatsky <senozhatsky@xxxxxxxxxxxx>
To: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: linux-mm@xxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
 mm/zpdesc.h   |  2 +-
 mm/zsmalloc.c | 24 ++++++------------------
 2 files changed, 7 insertions(+), 19 deletions(-)

diff --git a/mm/zpdesc.h b/mm/zpdesc.h
index b8258dc78548d..4fd81c2e80769 100644
--- a/mm/zpdesc.h
+++ b/mm/zpdesc.h
@@ -26,8 +26,8 @@
  * with memcg_data.
  *
  * Page flags used:
- * * PG_private identifies the first component page.
  * * PG_locked is used by page migration code.
+ * The first component page has zpdesc->zspage->first_zpdesc =3D=3D zpdesc
  */
 struct zpdesc {
 	unsigned long flags;
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 8204b76f78308..b87679757e7b1 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -290,11 +290,6 @@ struct zs_pool {
 	atomic_t compaction_in_progress;
 };
=20
-static inline void zpdesc_set_first(struct zpdesc *zpdesc)
-{
-	SetPagePrivate(zpdesc_page(zpdesc));
-}
-
 static inline void zpdesc_inc_zone_page_state(struct zpdesc *zpdesc)
 {
 	inc_zone_page_state(zpdesc_page(zpdesc), NR_ZSPAGES);
@@ -476,11 +471,6 @@ static void record_obj(unsigned long handle, unsigned =
long obj)
 	WRITE_ONCE(*(unsigned long *)handle, obj);
 }
=20
-static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc)
-{
-	return PagePrivate(zpdesc_page(zpdesc));
-}
-
 /* Protected by class->lock */
 static inline int get_zspage_inuse(struct zspage *zspage)
 {
@@ -496,7 +486,8 @@ static struct zpdesc *get_first_zpdesc(struct zspage *z=
spage)
 {
 	struct zpdesc *first_zpdesc =3D zspage->first_zpdesc;
=20
-	VM_BUG_ON_PAGE(!is_first_zpdesc(first_zpdesc), zpdesc_page(first_zpdesc))=
;
+	/* the first zpdesc must point back to this zspage */
+	VM_WARN_ON_ONCE_PAGE(first_zpdesc->zspage !=3D zspage, zpdesc_page(first_=
zpdesc));
 	return first_zpdesc;
 }
=20
@@ -833,7 +824,8 @@ static inline bool obj_allocated(struct zpdesc *zpdesc,=
 void *obj,
 	struct zspage *zspage =3D get_zspage(zpdesc);
=20
 	if (unlikely(ZsHugePage(zspage))) {
-		VM_BUG_ON_PAGE(!is_first_zpdesc(zpdesc), zpdesc_page(zpdesc));
+		/* only first zpdesc holds the handle */
+		VM_WARN_ON_ONCE_PAGE(zspage->first_zpdesc !=3D zpdesc, zpdesc_page(zpdes=
c));
 		handle =3D zpdesc->handle;
 	} else
 		handle =3D *(unsigned long *)obj;
@@ -848,9 +840,6 @@ static inline bool obj_allocated(struct zpdesc *zpdesc,=
 void *obj,
=20
 static void reset_zpdesc(struct zpdesc *zpdesc)
 {
-	struct page *page =3D zpdesc_page(zpdesc);
-
-	ClearPagePrivate(page);
 	zpdesc->zspage =3D NULL;
 	zpdesc->next =3D NULL;
 	/* PageZsmalloc is sticky until the page is freed to the buddy. */
@@ -1001,8 +990,8 @@ static void create_page_chain(struct size_class *class=
, struct zspage *zspage,
 	 * 1. all pages are linked together using zpdesc->next
 	 * 2. each sub-page point to zspage using zpdesc->zspage
 	 *
-	 * we set PG_private to identify the first zpdesc (i.e. no other zpdesc
-	 * has this flag set).
+	 * The first zpdesc has its zspage->first_zpdesc set to itself, no
+	 * other zpdesc has this set.
 	 */
 	for (i =3D 0; i < nr_zpdescs; i++) {
 		zpdesc =3D zpdescs[i];
@@ -1010,7 +999,6 @@ static void create_page_chain(struct size_class *class=
, struct zspage *zspage,
 		zpdesc->next =3D NULL;
 		if (i =3D=3D 0) {
 			zspage->first_zpdesc =3D zpdesc;
-			zpdesc_set_first(zpdesc);
 			if (unlikely(class->objs_per_zspage =3D=3D 1 &&
 					class->pages_per_zspage =3D=3D 1))
 				SetZsHugePage(zspage);
--=20
2.53.0




--=20
Best Regards,
Yan, Zi