[PATCH v10] mm: fix ASSERT_EXCLUSIVE_BITS by passing memdesc_flags_t by pointer

[PATCH v10] mm: fix ASSERT_EXCLUSIVE_BITS by passing memdesc_flags_t by pointer

[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 race is benign: nid/zone bits are set once at page init and never
overlap with PG_locked.  However, ASSERT_EXCLUSIVE_BITS() inside
memdesc_nid/zonenum() was checking a by-value copy of the flags word,
not the live page->flags, so it failed to annotate the real access.

Change memdesc_nid(), memdesc_zonenum(), memdesc_section(), and
memdesc_is_zone_device() to take a const memdesc_flags_t * and update
all callers to pass &page->flags / &folio->flags, so
ASSERT_EXCLUSIVE_BITS() operates on the actual shared word.

Guard the ASSERT_EXCLUSIVE_BITS() call in memdesc_zonenum() under
ZONES_WIDTH != 0 to avoid a zero-mask check on configs where the zone
field is absent.  memdesc_section() needs no such guard, since
SECTIONS_WIDTH is never 0 wherever SECTION_IN_PAGE_FLAGS is defined.
Under CONFIG_NUMA=n, memdesc_nid() itself is stubbed to "return 0"
instead of reading page->flags, since NODES_MASK is 0 and the check
can never fire; page_to_nid()/folio_nid() now just call memdesc_nid()
unconditionally and rely on that stub, instead of duplicating the
CONFIG_NUMA split at each call site.

Co-developed-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
Changelog:
v10:
According to the comments of David, drop the redundant CONFIG_NUMA split
in page_to_nid()/folio_nid() and remove the SECTIONS_WIDTH != 0 guard
around ASSERT_EXCLUSIVE_BITS() in memdesc_section().
v9:
Add the SECTIONS_WIDTH check to memdesc_section.
v8:
According to the comments of Andrew, include kcsan-checks.h in mm.h.
Incorporate David's patch that switch memdesc_nid(), memdesc_zonenum(),
memdesc_section() and memdesc_is_zone_device() to take a const
memdesc_flags_t * instead of using a per-accessor macro/call-site hack.
Update all callers accordingly and extend the same exclusive-bits check
to memdesc_section() and memdesc_is_zone_device(), guarded by
SECTIONS_WIDTH != 0 / reusing ZONES_WIDTH != 0 to avoid zero-mask checks
on configs without the corresponding field.
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/asm-generic/memory_model.h |  2 +-
 include/linux/mm.h                 | 28 ++++++++++++++++++++--------
 include/linux/mm_inline.h          |  4 ++--
 include/linux/mmzone.h             | 26 ++++++++++++++------------
 mm/page_alloc.c                    |  6 +++---
 mm/slab.h                          |  2 +-
 mm/sparse.c                        |  2 +-
 7 files changed, 42 insertions(+), 28 deletions(-)

diff --git a/include/asm-generic/memory_model.h b/include/asm-generic/memory_model.h
index fd74de50b054..c6b4eafaf4cb 100644
--- a/include/asm-generic/memory_model.h
+++ b/include/asm-generic/memory_model.h
@@ -53,7 +53,7 @@ static inline int pfn_valid(unsigned long pfn)
  */
 #define __page_to_pfn(pg)					\
 ({	const struct page *__pg = (pg);				\
-	int __sec = memdesc_section(__pg->flags);		\
+	int __sec = memdesc_section(&__pg->flags);		\
 	(unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec)));	\
 })
 
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 2101e5205fc0..01683d50b3cd 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -37,6 +37,7 @@
 #include <linux/bitmap.h>
 #include <linux/bitops.h>
 #include <linux/iommu-debug-pagealloc.h>
+#include <linux/kcsan-checks.h>
 
 struct mempolicy;
 struct anon_vma;
@@ -2286,22 +2287,32 @@ static inline int page_zone_id(struct page *page)
 }
 
 #ifdef NODE_NOT_IN_PAGE_FLAGS
-int memdesc_nid(memdesc_flags_t mdf);
+int memdesc_nid(const memdesc_flags_t *mdf);
 #else
-static inline int memdesc_nid(memdesc_flags_t mdf)
+#ifdef CONFIG_NUMA
+static inline int memdesc_nid(const memdesc_flags_t *mdf)
 {
-	return (mdf.f >> NODES_PGSHIFT) & NODES_MASK;
+	ASSERT_EXCLUSIVE_BITS(mdf->f, NODES_MASK << NODES_PGSHIFT);
+	return (mdf->f >> NODES_PGSHIFT) & NODES_MASK;
 }
+#else
+static inline int memdesc_nid(const memdesc_flags_t *mdf)
+{
+	return 0;
+}
+#endif
 #endif
 
 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);
+	return memdesc_nid(&folio->flags);
 }
 
 #ifdef CONFIG_NUMA_BALANCING
@@ -2541,12 +2552,13 @@ static inline void set_page_section(struct page *page, unsigned long section)
 	page->flags.f |= (section & SECTIONS_MASK) << SECTIONS_PGSHIFT;
 }
 
-static inline unsigned long memdesc_section(memdesc_flags_t mdf)
+static inline unsigned long memdesc_section(const memdesc_flags_t *mdf)
 {
-	return (mdf.f >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
+	ASSERT_EXCLUSIVE_BITS(mdf->f, SECTIONS_MASK << SECTIONS_PGSHIFT);
+	return (mdf->f >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
 }
 #else /* !SECTION_IN_PAGE_FLAGS */
-static inline unsigned long memdesc_section(memdesc_flags_t mdf)
+static inline unsigned long memdesc_section(const memdesc_flags_t *mdf)
 {
 	return 0;
 }
diff --git a/include/linux/mm_inline.h b/include/linux/mm_inline.h
index a8430a7ae054..efcddb9925ad 100644
--- a/include/linux/mm_inline.h
+++ b/include/linux/mm_inline.h
@@ -650,7 +650,7 @@ static inline bool vma_has_recency(const struct vm_area_struct *vma)
 static inline size_t num_pages_contiguous(struct page **pages, size_t nr_pages)
 {
 	struct page *cur_page = pages[0];
-	unsigned long section = memdesc_section(cur_page->flags);
+	unsigned long section = memdesc_section(&cur_page->flags);
 	size_t i;
 
 	for (i = 1; i < nr_pages; i++) {
@@ -660,7 +660,7 @@ static inline size_t num_pages_contiguous(struct page **pages, size_t nr_pages)
 		 * In unproblematic kernel configs, page_to_section() == 0 and
 		 * the whole check will get optimized out.
 		 */
-		if (memdesc_section(cur_page->flags) != section)
+		if (memdesc_section(&cur_page->flags) != section)
 			break;
 	}
 
diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index 242ec3cb8b52..0507193b3ae3 100644
--- a/include/linux/mmzone.h
+++ b/include/linux/mmzone.h
@@ -1272,31 +1272,33 @@ static inline bool zone_is_empty(const struct zone *zone)
 #define KASAN_TAG_MASK		((1UL << KASAN_TAG_WIDTH) - 1)
 #define ZONEID_MASK		((1UL << ZONEID_SHIFT) - 1)
 
-static inline enum zone_type memdesc_zonenum(memdesc_flags_t flags)
+static inline enum zone_type memdesc_zonenum(const memdesc_flags_t *flags)
 {
-	ASSERT_EXCLUSIVE_BITS(flags.f, ZONES_MASK << ZONES_PGSHIFT);
-	return (flags.f >> ZONES_PGSHIFT) & ZONES_MASK;
+#if ZONES_WIDTH != 0
+	ASSERT_EXCLUSIVE_BITS(flags->f, ZONES_MASK << ZONES_PGSHIFT);
+#endif
+	return (flags->f >> ZONES_PGSHIFT) & ZONES_MASK;
 }
 
 static inline enum zone_type page_zonenum(const struct page *page)
 {
-	return memdesc_zonenum(page->flags);
+	return memdesc_zonenum(&page->flags);
 }
 
 static inline enum zone_type folio_zonenum(const struct folio *folio)
 {
-	return memdesc_zonenum(folio->flags);
+	return memdesc_zonenum(&folio->flags);
 }
 
 #ifdef CONFIG_ZONE_DEVICE
-static inline bool memdesc_is_zone_device(memdesc_flags_t mdf)
+static inline bool memdesc_is_zone_device(const memdesc_flags_t *mdf)
 {
 	return memdesc_zonenum(mdf) == ZONE_DEVICE;
 }
 
 static inline struct dev_pagemap *page_pgmap(const struct page *page)
 {
-	VM_WARN_ON_ONCE_PAGE(!memdesc_is_zone_device(page->flags), page);
+	VM_WARN_ON_ONCE_PAGE(!memdesc_is_zone_device(&page->flags), page);
 	return page_folio(page)->pgmap;
 }
 
@@ -1311,9 +1313,9 @@ static inline struct dev_pagemap *page_pgmap(const struct page *page)
 static inline bool zone_device_pages_have_same_pgmap(const struct page *a,
 						     const struct page *b)
 {
-	if (memdesc_is_zone_device(a->flags) != memdesc_is_zone_device(b->flags))
+	if (memdesc_is_zone_device(&a->flags) != memdesc_is_zone_device(&b->flags))
 		return false;
-	if (!memdesc_is_zone_device(a->flags))
+	if (!memdesc_is_zone_device(&a->flags))
 		return true;
 	return page_pgmap(a) == page_pgmap(b);
 }
@@ -1321,7 +1323,7 @@ static inline bool zone_device_pages_have_same_pgmap(const struct page *a,
 extern void memmap_init_zone_device(struct zone *, unsigned long,
 				    unsigned long, struct dev_pagemap *);
 #else
-static inline bool memdesc_is_zone_device(memdesc_flags_t mdf)
+static inline bool memdesc_is_zone_device(const memdesc_flags_t *mdf)
 {
 	return false;
 }
@@ -1338,12 +1340,12 @@ static inline struct dev_pagemap *page_pgmap(const struct page *page)
 
 static inline bool is_zone_device_page(const struct page *page)
 {
-	return memdesc_is_zone_device(page->flags);
+	return memdesc_is_zone_device(&page->flags);
 }
 
 static inline bool folio_is_zone_device(const struct folio *folio)
 {
-	return memdesc_is_zone_device(folio->flags);
+	return memdesc_is_zone_device(&folio->flags);
 }
 
 static inline bool is_zone_movable_page(const struct page *page)
diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index 8a5c93fe1c41..a63733dac659 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -6998,15 +6998,15 @@ static void __free_contig_range_common(unsigned long pfn, unsigned long nr_pages
 			continue;
 		}
 
-		if (start && memdesc_section(page->flags) != start_sec) {
+		if (start && memdesc_section(&page->flags) != start_sec) {
 			free_prepared_contig_range(start, i - nr_start);
 			start = page;
 			nr_start = i;
-			start_sec = memdesc_section(page->flags);
+			start_sec = memdesc_section(&page->flags);
 		} else if (!start) {
 			start = page;
 			nr_start = i;
-			start_sec = memdesc_section(page->flags);
+			start_sec = memdesc_section(&page->flags);
 		}
 	}
 
diff --git a/mm/slab.h b/mm/slab.h
index 281a65233795..9ded319495a0 100644
--- a/mm/slab.h
+++ b/mm/slab.h
@@ -179,7 +179,7 @@ static inline void *slab_address(const struct slab *slab)
 
 static inline int slab_nid(const struct slab *slab)
 {
-	return memdesc_nid(slab->flags);
+	return memdesc_nid(&slab->flags);
 }
 
 static inline pg_data_t *slab_pgdat(const struct slab *slab)
diff --git a/mm/sparse.c b/mm/sparse.c
index 324213d8bdcb..058ef9300367 100644
--- a/mm/sparse.c
+++ b/mm/sparse.c
@@ -43,7 +43,7 @@ static u8 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned;
 static u16 section_to_node_table[NR_MEM_SECTIONS] __cacheline_aligned;
 #endif
 
-int memdesc_nid(memdesc_flags_t mdf)
+int memdesc_nid(const memdesc_flags_t *mdf)
 {
 	return section_to_node_table[memdesc_section(mdf)];
 }
-- 
2.43.0

Re: [PATCH v10] mm: fix ASSERT_EXCLUSIVE_BITS by passing memdesc_flags_t by pointer

[David Hildenbrand (Arm)]

On 7/8/26 04:15, Hui Zhu wrote:
> 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 race is benign: nid/zone bits are set once at page init and never
> overlap with PG_locked.  However, ASSERT_EXCLUSIVE_BITS() inside
> memdesc_nid/zonenum() was checking a by-value copy of the flags word,
> not the live page->flags, so it failed to annotate the real access.
> 
> Change memdesc_nid(), memdesc_zonenum(), memdesc_section(), and
> memdesc_is_zone_device() to take a const memdesc_flags_t * and update
> all callers to pass &page->flags / &folio->flags, so
> ASSERT_EXCLUSIVE_BITS() operates on the actual shared word.
> 
> Guard the ASSERT_EXCLUSIVE_BITS() call in memdesc_zonenum() under
> ZONES_WIDTH != 0 to avoid a zero-mask check on configs where the zone
> field is absent.  memdesc_section() needs no such guard, since
> SECTIONS_WIDTH is never 0 wherever SECTION_IN_PAGE_FLAGS is defined.
> Under CONFIG_NUMA=n, memdesc_nid() itself is stubbed to "return 0"
> instead of reading page->flags, since NODES_MASK is 0 and the check
> can never fire; page_to_nid()/folio_nid() now just call memdesc_nid()
> unconditionally and rely on that stub, instead of duplicating the
> CONFIG_NUMA split at each call site.
> 
> Co-developed-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
> ---
> Changelog:
> v10:
> According to the comments of David, drop the redundant CONFIG_NUMA split
> in page_to_nid()/folio_nid() and remove the SECTIONS_WIDTH != 0 guard
> around ASSERT_EXCLUSIVE_BITS() in memdesc_section().
> v9:
> Add the SECTIONS_WIDTH check to memdesc_section.
> v8:
> According to the comments of Andrew, include kcsan-checks.h in mm.h.
> Incorporate David's patch that switch memdesc_nid(), memdesc_zonenum(),
> memdesc_section() and memdesc_is_zone_device() to take a const
> memdesc_flags_t * instead of using a per-accessor macro/call-site hack.
> Update all callers accordingly and extend the same exclusive-bits check
> to memdesc_section() and memdesc_is_zone_device(), guarded by
> SECTIONS_WIDTH != 0 / reusing ZONES_WIDTH != 0 to avoid zero-mask checks
> on configs without the corresponding field.
> 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/asm-generic/memory_model.h |  2 +-
>  include/linux/mm.h                 | 28 ++++++++++++++++++++--------
>  include/linux/mm_inline.h          |  4 ++--
>  include/linux/mmzone.h             | 26 ++++++++++++++------------
>  mm/page_alloc.c                    |  6 +++---
>  mm/slab.h                          |  2 +-
>  mm/sparse.c                        |  2 +-
>  7 files changed, 42 insertions(+), 28 deletions(-)
> 
> diff --git a/include/asm-generic/memory_model.h b/include/asm-generic/memory_model.h
> index fd74de50b054..c6b4eafaf4cb 100644
> --- a/include/asm-generic/memory_model.h
> +++ b/include/asm-generic/memory_model.h
> @@ -53,7 +53,7 @@ static inline int pfn_valid(unsigned long pfn)
>   */
>  #define __page_to_pfn(pg)					\
>  ({	const struct page *__pg = (pg);				\
> -	int __sec = memdesc_section(__pg->flags);		\
> +	int __sec = memdesc_section(&__pg->flags);		\
>  	(unsigned long)(__pg - __section_mem_map_addr(__nr_to_section(__sec)));	\
>  })
>  
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 2101e5205fc0..01683d50b3cd 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -37,6 +37,7 @@
>  #include <linux/bitmap.h>
>  #include <linux/bitops.h>
>  #include <linux/iommu-debug-pagealloc.h>
> +#include <linux/kcsan-checks.h>
>  
>  struct mempolicy;
>  struct anon_vma;
> @@ -2286,22 +2287,32 @@ static inline int page_zone_id(struct page *page)
>  }
>  
>  #ifdef NODE_NOT_IN_PAGE_FLAGS
> -int memdesc_nid(memdesc_flags_t mdf);
> +int memdesc_nid(const memdesc_flags_t *mdf);
>  #else
> -static inline int memdesc_nid(memdesc_flags_t mdf)
> +#ifdef CONFIG_NUMA
> +static inline int memdesc_nid(const memdesc_flags_t *mdf)
>  {
> -	return (mdf.f >> NODES_PGSHIFT) & NODES_MASK;
> +	ASSERT_EXCLUSIVE_BITS(mdf->f, NODES_MASK << NODES_PGSHIFT);
> +	return (mdf->f >> NODES_PGSHIFT) & NODES_MASK;
>  }
> +#else
> +static inline int memdesc_nid(const memdesc_flags_t *mdf)
> +{
> +	return 0;
> +}
> +#endif
>  #endif
>  
>  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);
>  }

Nit: Why not keep it as

return memdesc_nid(&(PF_POISONED_CHECK(page)->flags));

Apart from that

Acked-by: David Hildenbrand (Arm) <david@kernel.org>

-- 
Cheers,

David