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 616b8371 ("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/03CE3A00-917C-48CC-8E1C-6A98713C817C@nvidia.com/

[zi.yan@sent.com: fix an error in migrate_misplaced_folio()]
  Link: https://lkml.kernel.org/r/20240326150031.569387-1-zi.yan@sent.com
Link: https://lkml.kernel.org/r/20240322193304.522496-1-zi.yan@sent.com
Fixes: 616b8371 ("mm: thp: enable thp migration in generic path")
Signed-off-by: Zi Yan <ziy@nvidia.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Huang, Ying <ying.huang@intel.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: SeongJae Park <sj@kernel.org>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Yin Fengwei <fengwei.yin@intel.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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