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* [PATCH v4] mm: assert exclusive nid/zonenum bits at the page/folio access sites
@ 2026-06-25  5:39 Hui Zhu
  2026-06-25  6:46 ` David Hildenbrand (Arm)
  2026-06-25  6:50 ` kernel test robot
  0 siblings, 2 replies; 3+ messages in thread
From: Hui Zhu @ 2026-06-25  5:39 UTC (permalink / raw)
  To: Andrew Morton, David Hildenbrand, Lorenzo Stoakes,
	Liam R. Howlett, Vlastimil Babka, Mike Rapoport,
	Suren Baghdasaryan, Michal Hocko, Kairui Song, Qi Zheng,
	Shakeel Butt, Barry Song, Axel Rasmussen, Yuanchu Xie, Wei Xu,
	linux-mm, linux-kernel
  Cc: Hui Zhu

From: Hui Zhu <zhuhui@kylinos.cn>

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 <zhuhui@kylinos.cn>
---
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


^ permalink raw reply related	[flat|nested] 3+ messages in thread
end of thread, other threads:[~2026-06-25  6:51 UTC | newest]

Thread overview: 3+ messages (download: mbox.gz follow: Atom feed
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2026-06-25  5:39 [PATCH v4] mm: assert exclusive nid/zonenum bits at the page/folio access sites Hui Zhu
2026-06-25  6:46 ` David Hildenbrand (Arm)
2026-06-25  6:50 ` kernel test robot

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