migrate_pages(): fix failure counting for THP on -ENOSYS

If THP or hugetlbfs page migration isn't supported, unmap_and_move() or
unmap_and_move_huge_page() will return -ENOSYS.  For THP, splitting will
be tried, but if splitting doesn't succeed, the THP will be left in "from"
list wrongly.  If some other pages are retried, the THP migration failure
will counted again.  This is fixed via moving the failure THP from "from"
to "ret_pages".

Another issue of the original code is that the unsupported failure
processing isn't consistent between THP and hugetlbfs page.  Make them
consistent in this patch to make the code easier to be understood too.

Link: https://lkml.kernel.org/r/20220817081408.513338-6-ying.huang@intel.com
Fixes: 5984fabb ("mm: move_pages: report the number of non-attempted pages")
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/migrate.c

@@ -1260,10 +1260,8 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 	 * tables or check whether the hugepage is pmd-based or not before
 	 * kicking migration.
 	 */
-	if (!hugepage_migration_supported(page_hstate(hpage))) {
-		list_move_tail(&hpage->lru, ret);
+	if (!hugepage_migration_supported(page_hstate(hpage)))
 		return -ENOSYS;
-	}
 
 	if (page_count(hpage) == 1) {
 		/* page was freed from under us. So we are done. */
@@ -1460,6 +1458,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 			 *		 page will be put back
 			 *	-EAGAIN: stay on the from list
 			 *	-ENOMEM: stay on the from list
+			 *	-ENOSYS: stay on the from list
 			 *	Other errno: put on ret_pages list then splice to
 			 *		     from list
 			 */
@@ -1489,6 +1488,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 				}
 
 				nr_failed_pages += nr_subpages;
+				list_move_tail(&page->lru, &ret_pages);
 				break;
 			case -ENOMEM:
 				/*