[PATCH v5] mm/migrate: split source folio if it is on deferred split list

[Zi Yan <zi.yan@sent.com>]

From: Zi Yan <ziy@nvidia.com>

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 616b8371539a6 ("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].

From v4:
1. Simplify _deferred_list check without locking and do not count as
   migration failures. (per Matthew Wilcox)

From v3:
1. Guarded deferred list code behind CONFIG_TRANSPARENT_HUGEPAGE to avoid
   compilation error (per SeongJae Park).

From v2:
1. Split the source folio instead of migrating it (per Matthew Wilcox)[2].

From v1:
1. Used dst to get correct deferred split list after migration
   (per Ryan Roberts).

[1]: https://lore.kernel.org/linux-mm/03CE3A00-917C-48CC-8E1C-6A98713C817C@nvidia.com/
[2]: https://lore.kernel.org/linux-mm/Ze_P6xagdTbcu1Kz@casper.infradead.org/

Fixes: 616b8371539a ("mm: thp: enable thp migration in generic path")
Signed-off-by: Zi Yan <ziy@nvidia.com>
---
 mm/migrate.c | 23 +++++++++++++++++++++++
 1 file changed, 23 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index ab9856f5931b..6bd9319624a3 100644
--- a/mm/migrate.c
+++ b/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, from) == 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

base-commit: 08a487ab26d541a3bd0adaee144f684b724d233b
-- 
2.43.0