Re: [PATCH v11] mm: fix ASSERT_EXCLUSIVE_BITS by passing memdesc_flags_t by pointer

[Andrew Morton <akpm@linux-foundation.org>]

On Wed,  8 Jul 2026 16:33:08 +0800 Hui Zhu <hui.zhu@linux.dev> 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.

Thanks.

> 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>
> Acked-by: David Hildenbrand (Arm) <david@kernel.org>
> ---
> Changelog:
> v11:
> According to the comments of David, simplify page_to_nid() to pass
> &(PF_POISONED_CHECK(page)->flags) directly.
> 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().

I'm having trouble comparing this changelogging with the actual v9->v11
diff?  "drop the redundant CONFIG_NUMA split"?


--- a/include/linux/mm.h~mm-fix-assert_exclusive_bits-by-passing-memdesc_flags_t-by-pointer-fix
+++ a/include/linux/mm.h
@@ -2303,29 +2303,15 @@ static inline int memdesc_nid(const memd
 #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);
+	return memdesc_nid(&(PF_POISONED_CHECK(page)->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 */
@@ -2566,9 +2552,7 @@ static inline void set_page_section(stru
 
 static inline unsigned long memdesc_section(const memdesc_flags_t *mdf)
 {
-#if SECTIONS_WIDTH != 0
 	ASSERT_EXCLUSIVE_BITS(mdf->f, SECTIONS_MASK << SECTIONS_PGSHIFT);
-#endif
 	return (mdf->f >> SECTIONS_PGSHIFT) & SECTIONS_MASK;
 }
 #else /* !SECTION_IN_PAGE_FLAGS */
_