From: Hui Zhu KCSAN reports a data race between page_to_nid()/folio_pgdat() 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 and zone id occupy fixed bit-ranges of page->flags that are set once at page init and never modified afterwards, so they can never overlap with the low PG_locked/PG_waiters bits touched by the folio lock path. ASSERT_EXCLUSIVE_BITS(mdf.f, ...) inside memdesc_nid()/memdesc_zonenum() checks a by-value copy of the flags word, not the actual shared page->flags/folio->flags being modified concurrently, so it doesn't reliably assert anything about the real race. Move the assertion to page_to_nid(), folio_nid(), page_zonenum() and folio_zonenum(), where flags is dereferenced directly from the page/folio. On CONFIG_NUMA=n, NODES_MASK is 0 and the old memdesc_nid() body folded to a constant, so page->flags/folio->flags was never actually read. ASSERT_EXCLUSIVE_BITS() is a real runtime check that can't be folded away, so doing it unconditionally would add a pointless read of page->flags/folio->flags and a check that can never fire. Keep page_to_nid()/folio_nid() as plain "return 0" static inline stubs under CONFIG_NUMA=n instead. Signed-off-by: Hui Zhu --- Changelog: v4: According to the comments of Andrew and Sashiko, set page_to_nid()/folio_nid() as static inline stubs returning 0 under CONFIG_NUMA=n. v3: According to the comments of Andrew and Sashiko, move ASSERT_EXCLUSIVE_BITS out of memdesc_nid()/memdesc_zonenum() into the page/folio call sites. 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 | 9 +++++++++ include/linux/mmzone.h | 3 ++- 2 files changed, 11 insertions(+), 1 deletion(-) diff --git a/include/linux/mm.h b/include/linux/mm.h index 485df9c2dbdd..56b39194605a 100644 --- a/include/linux/mm.h +++ b/include/linux/mm.h @@ -2294,15 +2294,24 @@ static inline int memdesc_nid(memdesc_flags_t mdf) } #endif +#ifdef CONFIG_NUMA static inline int page_to_nid(const struct page *page) { + ASSERT_EXCLUSIVE_BITS(PF_POISONED_CHECK(page)->flags, + NODES_MASK << NODES_PGSHIFT); return memdesc_nid(PF_POISONED_CHECK(page)->flags); } static inline int folio_nid(const struct folio *folio) { + ASSERT_EXCLUSIVE_BITS(folio->flags, + NODES_MASK << NODES_PGSHIFT); return memdesc_nid(folio->flags); } +#else +#define page_to_nid(page) (0) +#define folio_nid(folio) (0) +#endif #ifdef CONFIG_NUMA_BALANCING /* page access time bits needs to hold at least 4 seconds */ diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h index ca2712187147..56dffa966343 100644 --- a/include/linux/mmzone.h +++ b/include/linux/mmzone.h @@ -1274,17 +1274,18 @@ static inline bool zone_is_empty(const struct zone *zone) static inline enum zone_type memdesc_zonenum(memdesc_flags_t flags) { - ASSERT_EXCLUSIVE_BITS(flags.f, ZONES_MASK << ZONES_PGSHIFT); return (flags.f >> ZONES_PGSHIFT) & ZONES_MASK; } static inline enum zone_type page_zonenum(const struct page *page) { + ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT); return memdesc_zonenum(page->flags); } static inline enum zone_type folio_zonenum(const struct folio *folio) { + ASSERT_EXCLUSIVE_BITS(folio->flags, ZONES_MASK << ZONES_PGSHIFT); return memdesc_zonenum(folio->flags); } -- 2.43.0