Re: [PATCH v9] mm: fix ASSERT_EXCLUSIVE_BITS by passing memdesc_flags_t by pointer

["David Hildenbrand (Arm)" <david@kernel.org>]

On 6/30/26 09:08, 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() calls in memdesc_zonenum() and
> memdesc_section() under ZONES_WIDTH != 0 / SECTIONS_WIDTH != 0 to avoid
> a zero-mask check on configs where the corresponding field is absent.
> Under CONFIG_NUMA=n, stub out page_to_nid() and folio_nid() as plain
> "return 0" instead of reading page->flags when NODES_MASK is 0 and the
> check can never fire.
> 
> Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
> 
> 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:
> 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                 | 42 ++++++++++++++++++++++++------
>  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, 56 insertions(+), 28 deletions(-)
> 
> diff --git a/include/asm-generic/memory_model.h b/include/asm-generic/memory_model.h
> index efa6610acbc7..f8404bc7773c 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 485df9c2dbdd..60722c42a622 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;
> @@ -2286,23 +2287,45 @@ 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
> +
> +#ifdef CONFIG_NUMA
> +static inline int page_to_nid(const struct page *page)
> +{
> +	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 memdesc_nid(PF_POISONED_CHECK(page)->flags);
> +	return 0;
>  }
>  
>  static inline int folio_nid(const struct folio *folio)
>  {
> -	return memdesc_nid(folio->flags);
> +	return 0;
>  }

Why do we need two sets of helpers here as well? Shouldn't memdesc_nid() take
care of that already?

> +#endif
>  
>  #ifdef CONFIG_NUMA_BALANCING
>  /* page access time bits needs to hold at least 4 seconds */
> @@ -2541,12 +2564,15 @@ 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;
> +#if SECTIONS_WIDTH != 0

How can we have SECTION_IN_PAGE_FLAGS but !SECTIONS_WIDTH?

We have

#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
#define SECTION_IN_PAGE_FLAGS
#endif

And

#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
#define SECTIONS_WIDTH		SECTIONS_SHIFT
#else
...

> +	ASSERT_EXCLUSIVE_BITS(mdf->f, SECTIONS_MASK << SECTIONS_PGSHIFT);
> +#endif
> +	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 ca2712187147..e60dad546ca6 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;
>  }

Okay, ZONES_WIDTH can be 0 with "MAX_NR_ZONES < 2", and we'll always return 0.


Nothing else jumped at me.

-- 
Cheers,

David