migrate_pages(): fix failure counting for THP splitting

If THP is failed to be migrated, it may be split and retry.  But after
splitting, the head page will be left in "from" list, although THP
migration failure has been counted already.  If the head page is failed to
be migrated too, the failure will be counted twice incorrectly.  So this
is fixed in this patch via moving the head page of THP after splitting to
"thp_split_pages" too.

Link: https://lkml.kernel.org/r/20220817081408.513338-7-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>
Reviewed-by: Oscar Salvador <osalvador@suse.de>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/migrate.c

@@ -1374,6 +1374,8 @@ static inline int try_split_thp(struct page *page, struct list_head *split_pages
 	lock_page(page);
 	rc = split_huge_page_to_list(page, split_pages);
 	unlock_page(page);
+	if (!rc)
+		list_move_tail(&page->lru, split_pages);
 
 	return rc;
 }
@@ -1433,7 +1435,6 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 		thp_retry = 0;
 
 		list_for_each_entry_safe(page, page2, from, lru) {
-retry:
 			/*
 			 * THP statistics is based on the source huge page.
 			 * Capture required information that might get lost
@@ -1469,10 +1470,9 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 			 * retry on the same page with the THP split
 			 * to base pages.
 			 *
-			 * Head page is retried immediately and tail
-			 * pages are added to the tail of the list so
-			 * we encounter them after the rest of the list
-			 * is processed.
+			 * Sub-pages are put in thp_split_pages, and
+			 * we will migrate them after the rest of the
+			 * list is processed.
 			 */
 			case -ENOSYS:
 				/* THP migration is unsupported */
@@ -1480,7 +1480,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 					nr_thp_failed++;
 					if (!try_split_thp(page, &thp_split_pages)) {
 						nr_thp_split++;
-						goto retry;
+						break;
 					}
 				/* Hugetlb migration is unsupported */
 				} else if (!no_subpage_counting) {
@@ -1500,7 +1500,7 @@ int migrate_pages(struct list_head *from, new_page_t get_new_page,
 					/* THP NUMA faulting doesn't split THP to retry. */
 					if (!nosplit && !try_split_thp(page, &thp_split_pages)) {
 						nr_thp_split++;
-						goto retry;
+						break;
 					}
 				} else if (!no_subpage_counting) {
 					nr_failed++;