Re: [PATCH v5] mm: assert exclusive nid/zonenum bits at the page/folio access sites

[David Hildenbrand (Arm)]

On 6/25/26 09:18, 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()
> 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@xxxxxxxxxx>
> Acked-by: David Hildenbrand (Arm) <david@xxxxxxxxxx>
> ---
> Changelog:
> 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     | 23 ++++++++++++++++++++++-
>  include/linux/mmzone.h |  7 ++++++-
>  2 files changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 485df9c2dbdd..772bd1fc6fe7 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2294,15 +2294,36 @@ static inline int memdesc_nid(memdesc_flags_t mdf)
>  }
>  #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);
> +
> +#ifndef NODE_NOT_IN_PAGE_FLAGS
> +	ASSERT_EXCLUSIVE_BITS(p->flags, NODES_MASK << NODES_PGSHIFT);
> +#endif
> +	return memdesc_nid(p->flags);
>  }
>  
>  static inline int folio_nid(const struct folio *folio)
>  {
> +#ifndef NODE_NOT_IN_PAGE_FLAGS
> +	ASSERT_EXCLUSIVE_BITS(folio->flags,
> +			      NODES_MASK << NODES_PGSHIFT);
> +#endif47

This is getting ugly, really. We're leaking implementation details from
memdesc_nid() into folio_nid().

Maybe just turn memdesc_nid() into a macro where we can just do that check
internally? Not the best thing in this world, but better than this here.

-- 
Cheers,

David