[PATCH v7] mm: assert exclusive nid/zonenum bits at the page/folio access sites

[Hui Zhu]

From: Hui Zhu <zhuhui@xxxxxxxxxx>

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()
used to check a by-value copy of the flags word, not the actual shared
page->flags/folio->flags being modified concurrently, so it didn't
reliably assert anything about the real race.

For zonenum, move the assertion out of memdesc_zonenum() into
page_zonenum() and folio_zonenum(), where flags is dereferenced
directly from the page/folio.

For nid, turn memdesc_nid() into a macro instead, so the mdf argument
is expanded as the caller's own flags expression
(PF_POISONED_CHECK(page)->flags or folio->flags) rather than copied
into a function parameter, letting ASSERT_EXCLUSIVE_BITS() check the
real page->flags/folio->flags directly.

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@xxxxxxxxxx>
---
Changelog:
v7:
According to the comments of Sashiko, restrict the memdesc_nid() macro
to CONFIG_NUMA, keeping a plain "return 0" static inline stub otherwise,
and re-add a local page pointer in page_to_nid() to avoid evaluating
PF_POISONED_CHECK(page) twice.
v6:
According to the comments of David, turn memdesc_nid() from a static
inline function into a macro so ASSERT_EXCLUSIVE_BITS() can check the
caller's page->flags/folio->flags directly.
v5:
According to the comments of Sashiko, guard the ASSERT_EXCLUSIVE_BITS()
calls with #ifndef NODE_NOT_IN_PAGE_FLAGS (for nid) and #if
ZONES_WIDTH != 0 (for zonenum).
According to the comments of David, avoid calling
PF_POISONED_CHECK(page) twice in page_to_nid().
According to the warning of lkp, switch the CONFIG_NUMA=n
page_to_nid()/folio_nid() stubs from macros to static inline functions.
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     | 25 +++++++++++++++++++++++--
 include/linux/mmzone.h |  7 ++++++-
 2 files changed, 29 insertions(+), 3 deletions(-)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 485df9c2dbdd..63fcf277b675 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -2288,21 +2288,42 @@ static inline int page_zone_id(struct page *page)
 #ifdef NODE_NOT_IN_PAGE_FLAGS
 int memdesc_nid(memdesc_flags_t mdf);
 #else
+#ifdef CONFIG_NUMA
+#define memdesc_nid(mdf)						\
+({									\
+	ASSERT_EXCLUSIVE_BITS(mdf.f, NODES_MASK << NODES_PGSHIFT);	\
+	(int)((mdf.f >> NODES_PGSHIFT) & NODES_MASK);			\
+})
+#else
 static inline int memdesc_nid(memdesc_flags_t mdf)
 {
-	return (mdf.f >> NODES_PGSHIFT) & NODES_MASK;
+	return 0;
 }
 #endif
 
+#ifdef CONFIG_NUMA
 static inline int page_to_nid(const struct page *page)
 {
-	return memdesc_nid(PF_POISONED_CHECK(page)->flags);
+	const struct page *p = PF_POISONED_CHECK(page);
+
+	return memdesc_nid(p->flags);
 }
 
 static inline int folio_nid(const struct folio *folio)
 {
 	return memdesc_nid(folio->flags);
 }
+#else
+static inline int page_to_nid(const struct page *page)
+{
+	return 0;
+}
+
+static inline int folio_nid(const struct folio *folio)
+{
+	return 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..1b4336098113 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1274,17 +1274,22 @@ 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)
 {
+#if ZONES_WIDTH != 0
+	ASSERT_EXCLUSIVE_BITS(page->flags, ZONES_MASK << ZONES_PGSHIFT);
+#endif
 	return memdesc_zonenum(page->flags);
 }
 
 static inline enum zone_type folio_zonenum(const struct folio *folio)
 {
+#if ZONES_WIDTH != 0
+	ASSERT_EXCLUSIVE_BITS(folio->flags, ZONES_MASK << ZONES_PGSHIFT);
+#endif
 	return memdesc_zonenum(folio->flags);
 }
 
-- 
2.43.0