[PATCH RFC 01/14] mm/zsmalloc: replace PG_private with pointer comparison

zsmalloc uses PG_private to indicate first zpdesc in the zspage chain.
Replace it with zpdesc->zspage->first_zpdesc == zpdesc. The check,
is_first_zpdesc(), is only used in VM_BUG_ON(), so performance impact
should be negligible.

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 <[email protected]>
To: Minchan Kim <[email protected]>
To: Sergey Senozhatsky <[email protected]>
To: Andrew Morton <[email protected]>
Cc: [email protected]
Cc: [email protected]
---
 mm/zpdesc.h   |  2 +-
 mm/zsmalloc.c | 15 +++------------
 2 files changed, 4 insertions(+), 13 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 == zpdesc
  */
 struct zpdesc {
 	unsigned long flags;
diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 8204b76f78308..e8ef227624efa 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -290,11 +290,6 @@ struct zs_pool {
 	atomic_t compaction_in_progress;
 };
 
-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);
@@ -478,7 +473,7 @@ static void record_obj(unsigned long handle, unsigned long obj)
 
 static inline bool __maybe_unused is_first_zpdesc(struct zpdesc *zpdesc)
 {
-	return PagePrivate(zpdesc_page(zpdesc));
+	return zpdesc->zspage->first_zpdesc == zpdesc;
 }
 
 /* Protected by class->lock */
@@ -848,9 +843,6 @@ static inline bool obj_allocated(struct zpdesc *zpdesc, void *obj,
 
 static void reset_zpdesc(struct zpdesc *zpdesc)
 {
-	struct page *page = zpdesc_page(zpdesc);
-
-	ClearPagePrivate(page);
 	zpdesc->zspage = NULL;
 	zpdesc->next = NULL;
 	/* PageZsmalloc is sticky until the page is freed to the buddy. */
@@ -1001,8 +993,8 @@ 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 = 0; i < nr_zpdescs; i++) {
 		zpdesc = zpdescs[i];
@@ -1010,7 +1002,6 @@ static void create_page_chain(struct size_class *class, struct zspage *zspage,
 		zpdesc->next = NULL;
 		if (i == 0) {
 			zspage->first_zpdesc = zpdesc;
-			zpdesc_set_first(zpdesc);
 			if (unlikely(class->objs_per_zspage == 1 &&
 					class->pages_per_zspage == 1))
 				SetZsHugePage(zspage);

-- 
2.53.0