mm/rmap: Constify the rmap_walk_control argument

The rmap walking functions do not modify the rmap_walk_control, and
page_idle_clear_pte_refs() takes advantage of that to move construction
of the rmap_walk_control to compile time.  This lets us remove an
unclean cast.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

include/linux/ksm.h

@@ -51,7 +51,7 @@ static inline void ksm_exit(struct mm_struct *mm)
 struct page *ksm_might_need_to_copy(struct page *page,
 			struct vm_area_struct *vma, unsigned long address);
 
-void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc);
+void rmap_walk_ksm(struct folio *folio, const struct rmap_walk_control *rwc);
 void folio_migrate_ksm(struct folio *newfolio, struct folio *folio);
 
 #else  /* !CONFIG_KSM */
@@ -79,7 +79,7 @@ static inline struct page *ksm_might_need_to_copy(struct page *page,
 }
 
 static inline void rmap_walk_ksm(struct folio *folio,
-			struct rmap_walk_control *rwc)
+			const struct rmap_walk_control *rwc)
 {
 }
 

include/linux/rmap.h

@@ -292,8 +292,8 @@ struct rmap_walk_control {
 	bool (*invalid_vma)(struct vm_area_struct *vma, void *arg);
 };
 
-void rmap_walk(struct folio *folio, struct rmap_walk_control *rwc);
-void rmap_walk_locked(struct folio *folio, struct rmap_walk_control *rwc);
+void rmap_walk(struct folio *folio, const struct rmap_walk_control *rwc);
+void rmap_walk_locked(struct folio *folio, const struct rmap_walk_control *rwc);
 
 #else	/* !CONFIG_MMU */
 

mm/ksm.c

@@ -2588,7 +2588,7 @@ struct page *ksm_might_need_to_copy(struct page *page,
 	return new_page;
 }
 
-void rmap_walk_ksm(struct folio *folio, struct rmap_walk_control *rwc)
+void rmap_walk_ksm(struct folio *folio, const struct rmap_walk_control *rwc)
 {
 	struct stable_node *stable_node;
 	struct rmap_item *rmap_item;

mm/page_idle.c

@@ -103,7 +103,7 @@ static void page_idle_clear_pte_refs(struct page *page)
 	if (need_lock && !folio_trylock(folio))
 		return;
 
-	rmap_walk(folio, (struct rmap_walk_control *)&rwc);
+	rmap_walk(folio, &rwc);
 
 	if (need_lock)
 		folio_unlock(folio);

mm/rmap.c

@@ -2173,7 +2173,7 @@ void __put_anon_vma(struct anon_vma *anon_vma)
 }
 
 static struct anon_vma *rmap_walk_anon_lock(struct folio *folio,
-					struct rmap_walk_control *rwc)
+					const struct rmap_walk_control *rwc)
 {
 	struct anon_vma *anon_vma;
 
@@ -2203,8 +2203,8 @@ static struct anon_vma *rmap_walk_anon_lock(struct folio *folio,
  * Find all the mappings of a page using the mapping pointer and the vma chains
  * contained in the anon_vma struct it points to.
  */
-static void rmap_walk_anon(struct folio *folio, struct rmap_walk_control *rwc,
-		bool locked)
+static void rmap_walk_anon(struct folio *folio,
+		const struct rmap_walk_control *rwc, bool locked)
 {
 	struct anon_vma *anon_vma;
 	pgoff_t pgoff_start, pgoff_end;
@@ -2251,8 +2251,8 @@ static void rmap_walk_anon(struct folio *folio, struct rmap_walk_control *rwc,
  * Find all the mappings of a page using the mapping pointer and the vma chains
  * contained in the address_space struct it points to.
  */
-static void rmap_walk_file(struct folio *folio, struct rmap_walk_control *rwc,
-		bool locked)
+static void rmap_walk_file(struct folio *folio,
+		const struct rmap_walk_control *rwc, bool locked)
 {
 	struct address_space *mapping = folio_mapping(folio);
 	pgoff_t pgoff_start, pgoff_end;
@@ -2294,7 +2294,7 @@ static void rmap_walk_file(struct folio *folio, struct rmap_walk_control *rwc,
 		i_mmap_unlock_read(mapping);
 }
 
-void rmap_walk(struct folio *folio, struct rmap_walk_control *rwc)
+void rmap_walk(struct folio *folio, const struct rmap_walk_control *rwc)
 {
 	if (unlikely(folio_test_ksm(folio)))
 		rmap_walk_ksm(folio, rwc);
@@ -2305,7 +2305,7 @@ void rmap_walk(struct folio *folio, struct rmap_walk_control *rwc)
 }
 
 /* Like rmap_walk, but caller holds relevant rmap lock */
-void rmap_walk_locked(struct folio *folio, struct rmap_walk_control *rwc)
+void rmap_walk_locked(struct folio *folio, const struct rmap_walk_control *rwc)
 {
 	/* no ksm support for now */
 	VM_BUG_ON_FOLIO(folio_test_ksm(folio), folio);