[PATCH v3] mm: annotate data-race in cpu_needs_drain()

[Xuewen Wang]

KCSAN reports a data-race when cpu_needs_drain() reads another CPU's
per-cpu folio_batch->nr without locking, while the owning CPU writes
to it via folio_batch_add().

Reading a slightly stale value is harmless -- cpu_needs_drain() only
decides whether to schedule a drain, and the next iteration of
__lru_add_drain_all() will re-check. Use data_race() to annotate
the intentional race.

Signed-off-by: Xuewen Wang <wangxuewen@xxxxxxxxxx>
---
Changes in v3:
- Wrap the entire || expression in a single data_race() instead of wrapping
  each folio_batch_count() call individually, as suggested by Pedro and Lorenzo.
  This is equally effective and more readable.
- Remove data_race() from need_mlock_drain(), as it is now covered by the data_race()
  in cpu_needs_drain().
v2:
  https://lore.kernel.org/all/20260625065153.1581419-1-wangxuewen@xxxxxxxxxx/
v1:
  https://lore.kernel.org/all/20260624092606.1083449-1-wangxuewen@xxxxxxxxxx/
---
 mm/swap.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 588f50d8f1a8..46ea207e0624 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu)
 	struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
 
 	/* Check these in order of likelihood that they're not zero */
-	return folio_batch_count(&fbatches->lru_add) ||
+	return data_race(folio_batch_count(&fbatches->lru_add) ||
 		folio_batch_count(&fbatches->lru_move_tail) ||
 		folio_batch_count(&fbatches->lru_deactivate_file) ||
 		folio_batch_count(&fbatches->lru_deactivate) ||
 		folio_batch_count(&fbatches->lru_lazyfree) ||
 		folio_batch_count(&fbatches->lru_activate) ||
-		need_mlock_drain(cpu) ||
+		need_mlock_drain(cpu)) ||
 		has_bh_in_lru(cpu, NULL);
 }
 
-- 
2.25.1