From f078b0a0727c723cba4e55b331571f9804d459ca Mon Sep 17 00:00:00 2001
From: Xuewen Wang <wangxuewen@kylinos.cn>
Date: Fri, 26 Jun 2026 13:37:00 +0800
Subject: [PATCH] mm: annotate data-race in cpu_needs_drain()

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.

[akpm@linux-foundation.org: reindent cpu_needs_drain, per David & Lorenzo]
Link: https://lore.kernel.org/20260626053700.2036899-1-wangxuewen@kylinos.cn
Signed-off-by: Xuewen Wang <wangxuewen@kylinos.cn>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>
Cc: Axel Rasmussen <axelrasmussen@google.com>
Cc: Baoquan He <baoquan.he@linux.dev>
Cc: Barry Song <baohua@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Kairui Song <kasong@tencent.com>
Cc: Kemeng Shi <shikemeng@huaweicloud.com>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Nhat Pham <nphamcs@gmail.com>
Cc: Shakeel Butt <shakeel.butt@linux.dev>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Wei Xu <weixugc@google.com>
Cc: Yuanchu Xie <yuanchu@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
---
 mm/swap.c | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/mm/swap.c b/mm/swap.c
index 460e56370b3c..58e4eff698cc 100644
--- a/mm/swap.c
+++ b/mm/swap.c
@@ -831,13 +831,13 @@ 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) ||
-		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) ||
+	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)) ||
 		has_bh_in_lru(cpu, NULL);
 }