mm/migrate: split source folio if it is on deferred split list

If the source folio is on deferred split list, it is likely some subpages
are not used.  Split it before migration to avoid migrating unused
subpages.

Commit 616b837 ("mm: thp: enable thp migration in generic path") did
not check if a THP is on deferred split list before migration, thus, the
destination THP is never put on deferred split list even if the source THP
might be.  The opportunity of reclaiming free pages in a partially mapped
THP during deferred list scanning is lost, but no other harmful
consequence is present[1].

[1]: https://lore.kernel.org/linux-mm/[email protected]/

[[email protected]: fix an error in migrate_misplaced_folio()]
  Link: https://lkml.kernel.org/r/[email protected]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: 616b837 ("mm: thp: enable thp migration in generic path")
Signed-off-by: Zi Yan <[email protected]>
Tested-by: Baolin Wang <[email protected]>
Reviewed-by: Baolin Wang <[email protected]>
Acked-by: David Hildenbrand <[email protected]>
Cc: Huang, Ying <[email protected]>
Cc: Kirill A. Shutemov <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: SeongJae Park <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yin Fengwei <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: x-y-z

mm/migrate.c

@@ -1652,6 +1652,29 @@ static int migrate_pages_batch(struct list_head *from,
 
 			cond_resched();
 
+			/*
+			 * The rare folio on the deferred split list should
+			 * be split now. It should not count as a failure.
+			 * Only check it without removing it from the list.
+			 * Since the folio can be on deferred_split_scan()
+			 * local list and removing it can cause the local list
+			 * corruption. Folio split process below can handle it
+			 * with the help of folio_ref_freeze().
+			 *
+			 * nr_pages > 2 is needed to avoid checking order-1
+			 * page cache folios. They exist, in contrast to
+			 * non-existent order-1 anonymous folios, and do not
+			 * use _deferred_list.
+			 */
+			if (nr_pages > 2 &&
+			   !list_empty(&folio->_deferred_list)) {
+				if (try_split_folio(folio, split_folios) == 0) {
+					stats->nr_thp_split += is_thp;
+					stats->nr_split++;
+					continue;
+				}
+			}
+
 			/*
 			 * Large folio migration might be unsupported or
 			 * the allocation might be failed so we should retry