[PATCH 1/2] mm: Retry migration earlier upon refcount mismatch

[Dev Jain <dev.jain@arm.com>]

As already being done in __migrate_folio(), wherein we backoff if the
folio refcount is wrong, make this check during the unmapping phase, upon
the failure of which, the original state of the PTEs will be restored and
the folio lock will be dropped via migrate_folio_undo_src(), any racing
thread will make progress and migration will be retried.

Signed-off-by: Dev Jain <dev.jain@arm.com>
---
 mm/migrate.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/mm/migrate.c b/mm/migrate.c
index e7296c0fb5d5..477acf996951 100644
--- a/mm/migrate.c
+++ b/mm/migrate.c
@@ -1250,6 +1250,15 @@ static int migrate_folio_unmap(new_folio_t get_new_folio,
 	}
 
 	if (!folio_mapped(src)) {
+		/*
+		 * Someone may have changed the refcount and maybe sleeping
+		 * on the folio lock. In case of refcount mismatch, bail out,
+		 * let the system make progress and retry.
+		 */
+		struct address_space *mapping = folio_mapping(src);
+
+		if (folio_ref_count(src) != folio_expected_refs(mapping, src))
+			goto out;
 		__migrate_folio_record(dst, old_page_state, anon_vma);
 		return MIGRATEPAGE_UNMAP;
 	}
-- 
2.30.2