[PATCH v2] mm: avoid KCSAN false positive in memdesc_nid()

[Hui Zhu <hui.zhu@linux.dev>]

From: Hui Zhu <zhuhui@kylinos.cn>

KCSAN reports a data race between page_to_nid()/folio_nid() reading
page->flags and folio_trylock()/folio_lock() concurrently doing
test_and_set_bit_lock(PG_locked, ...) on the same word, e.g.:

  BUG: KCSAN: data-race in __lruvec_stat_mod_folio / shmem_get_folio_gfp

The node id occupies a fixed bit-range of page->flags that is set
once at page init and never modified afterwards, so it can never
overlap with the low PG_locked/PG_waiters bits touched by the folio
lock path.

Use ASSERT_EXCLUSIVE_BITS() in memdesc_nid() to scope the exemption
to just the node-id bits, consistent with how memdesc_zonenum()
already handles the same class of race for the zone-id bits.

Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
Changelog:
v2:
According to the comments of David, remove useless comments and use
ASSERT_EXCLUSIVE_BITS() in memdesc_nid() instead of data_race() in
page_to_nid().

 include/linux/mm.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 485df9c2dbdd..7518d6364a00 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2290,6 +2290,7 @@ int memdesc_nid(memdesc_flags_t mdf);
 #else
 static inline int memdesc_nid(memdesc_flags_t mdf)
 {
+	ASSERT_EXCLUSIVE_BITS(mdf.f, NODES_MASK << NODES_PGSHIFT);
 	return (mdf.f >> NODES_PGSHIFT) & NODES_MASK;
 }
 #endif
-- 
2.43.0