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

[Leon Hwang]

On 26/6/26 11:20, Hui Zhu wrote:
> 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);
>  }
>  

Better to factor out a common macro alongside a comment for these two '#if'?

Thanks,
Leon