mm: migrate: clean up migrate_prep{_local}

The migrate_prep{_local} never fails, so it is pointless to have return
value and check the return value.

Link: https://lkml.kernel.org/r/20201113205359.556831-5-shy828301@gmail.comSigned-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Jan Kara <jack@suse.cz>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Song Liu <songliubraving@fb.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/migrate.h

@@ -45,8 +45,8 @@ extern struct page *alloc_migration_target(struct page *page, unsigned long priv
 extern int isolate_movable_page(struct page *page, isolate_mode_t mode);
 extern void putback_movable_page(struct page *page);
 
-extern int migrate_prep(void);
-extern int migrate_prep_local(void);
+extern void migrate_prep(void);
+extern void migrate_prep_local(void);
 extern void migrate_page_states(struct page *newpage, struct page *page);
 extern void migrate_page_copy(struct page *newpage, struct page *page);
 extern int migrate_huge_page_move_mapping(struct address_space *mapping,

mm/mempolicy.c

@@ -1114,9 +1114,7 @@ int do_migrate_pages(struct mm_struct *mm, const nodemask_t *from,
 	int err;
 	nodemask_t tmp;
 
-	err = migrate_prep();
-	if (err)
-		return err;
+	migrate_prep();
 
 	mmap_read_lock(mm);
 
@@ -1315,9 +1313,7 @@ static long do_mbind(unsigned long start, unsigned long len,
 
 	if (flags & (MPOL_MF_MOVE | MPOL_MF_MOVE_ALL)) {
 
-		err = migrate_prep();
-		if (err)
-			goto mpol_out;
+		migrate_prep();
 	}
 	{
 		NODEMASK_SCRATCH(scratch);

mm/migrate.c

@@ -62,7 +62,7 @@
  * to be migrated using isolate_lru_page(). If scheduling work on other CPUs is
  * undesirable, use migrate_prep_local()
  */
-int migrate_prep(void)
+void migrate_prep(void)
 {
 	/*
 	 * Clear the LRU lists so pages can be isolated.
@@ -71,16 +71,12 @@ int migrate_prep(void)
 	 * pages that may be busy.
 	 */
 	lru_add_drain_all();
-
-	return 0;
 }
 
 /* Do the necessary work of migrate_prep but not if it involves other CPUs */
-int migrate_prep_local(void)
+void migrate_prep_local(void)
 {
 	lru_add_drain();
-
-	return 0;
 }
 
 int isolate_movable_page(struct page *page, isolate_mode_t mode)