ksm: use a folio inside cmp_and_merge_page()

This removes the last call to page_stable_node(), so delete the wrapper. 
It also removes a call to trylock_page() and saves a call to
compound_head(), as well as removing a reference to folio->page.

Link: https://lkml.kernel.org/r/20250916181219.2400258-1-willy@infradead.org
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Longlong Xia <xialonglong@kylinos.cn>
Cc: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: Matthew Wilcox (Oracle)

mm/ksm.c

@@ -1061,11 +1061,6 @@ struct ksm_stable_node *folio_stable_node(const struct folio *folio)
 	return folio_test_ksm(folio) ? folio_raw_mapping(folio) : NULL;
 }
 
-static inline struct ksm_stable_node *page_stable_node(struct page *page)
-{
-	return folio_stable_node(page_folio(page));
-}
-
 static inline void folio_set_stable_node(struct folio *folio,
 					 struct ksm_stable_node *stable_node)
 {
@@ -2225,6 +2220,7 @@ static void stable_tree_append(struct ksm_rmap_item *rmap_item,
  */
 static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_item)
 {
+	struct folio *folio = page_folio(page);
 	struct ksm_rmap_item *tree_rmap_item;
 	struct page *tree_page = NULL;
 	struct ksm_stable_node *stable_node;
@@ -2233,7 +2229,7 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
 	int err;
 	bool max_page_sharing_bypass = false;
 
-	stable_node = page_stable_node(page);
+	stable_node = folio_stable_node(folio);
 	if (stable_node) {
 		if (stable_node->head != &migrate_nodes &&
 		    get_kpfn_nid(READ_ONCE(stable_node->kpfn)) !=
@@ -2272,7 +2268,7 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
 
 	/* Start by searching for the folio in the stable tree */
 	kfolio = stable_tree_search(page);
-	if (&kfolio->page == page && rmap_item->head == stable_node) {
+	if (kfolio == folio && rmap_item->head == stable_node) {
 		folio_put(kfolio);
 		return;
 	}
@@ -2353,10 +2349,11 @@ static void cmp_and_merge_page(struct page *page, struct ksm_rmap_item *rmap_ite
 			 * the page is locked, it is better to skip it and
 			 * perhaps try again later.
 			 */
-			if (!trylock_page(page))
+			if (!folio_trylock(folio))
 				return;
 			split_huge_page(page);
-			unlock_page(page);
+			folio = page_folio(page);
+			folio_unlock(folio);
 		}
 	}
 }