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

[Hui Zhu]

From: Hui Zhu <zhuhui@xxxxxxxxxx>

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

  BUG: KCSAN: data-race in __lruvec_stat_mod_folio / shmem_get_folio_gfp

The node id occupies a fixed, high bit-range of page->flags that is
set once when the page is initialized and never modified afterwards,
so it can never overlap with the low PG_locked/PG_waiters bits touched
by the folio lock path. The race is therefore harmless: page_to_nid()
always returns a consistent value regardless of how the read
interleaves with the lock bit ops.

Wrap the flags read with data_race() to tell KCSAN this race is
intentional and benign, consistent with how page->page_type is
already annotated for similar packed-field accesses.

Signed-off-by: Hui Zhu <zhuhui@xxxxxxxxxx>
---
 include/linux/mm.h | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 485df9c2dbdd..122d3b39369f 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2296,7 +2296,14 @@ static inline int memdesc_nid(memdesc_flags_t mdf)
 
 static inline int page_to_nid(const struct page *page)
 {
-	return memdesc_nid(PF_POISONED_CHECK(page)->flags);
+	/*
+	 * The node id occupies a fixed high bit-range of page->flags
+	 * that is set once at page init and never changed afterwards.
+	 * It cannot overlap with the low PG_locked/PG_waiters bits
+	 * that folio_lock()/folio_unlock() concurrently update, so
+	 * this data race is benign.
+	 */
+	return memdesc_nid(data_race(PF_POISONED_CHECK(page)->flags));
 }
 
 static inline int folio_nid(const struct folio *folio)
-- 
2.43.0