mm/migrate: correct thp migration stats

PageTransHuge returns true for both thp and hugetlb, so thp stats was
counting both thp and hugetlb migrations.  Exclude hugetlb migration by
setting is_thp variable right.

Clean up thp handling code too when we are there.

Fixes: 1a5bae25e3 ("mm/vmstat: add events for THP migration without split")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Daniel Jordan <daniel.m.jordan@oracle.com>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Link: https://lkml.kernel.org/r/20200917210413.1462975-1-zi.yan@sent.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/migrate.c

@@ -1446,7 +1446,7 @@ retry:
 			 * Capture required information that might get lost
 			 * during migration.
 			 */
-			is_thp = PageTransHuge(page);
+			is_thp = PageTransHuge(page) && !PageHuge(page);
 			nr_subpages = thp_nr_pages(page);
 			cond_resched();
 
@@ -1472,7 +1472,7 @@ retry:
 				 * we encounter them after the rest of the list
 				 * is processed.
 				 */
-				if (PageTransHuge(page) && !PageHuge(page)) {
+				if (is_thp) {
 					lock_page(page);
 					rc = split_huge_page_to_list(page, from);
 					unlock_page(page);
@@ -1481,8 +1481,7 @@ retry:
 						nr_thp_split++;
 						goto retry;
 					}
-				}
-				if (is_thp) {
+
 					nr_thp_failed++;
 					nr_failed += nr_subpages;
 					goto out;