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

[David Hildenbrand (Arm)]

On 6/25/26 14:07, Lorenzo Stoakes wrote:
> On Thu, Jun 25, 2026 at 01:53:14PM +0200, David Hildenbrand (Arm) wrote:
>> 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.
> 
> Could also do:
> 
> 	if (!IS_ENABLED(NODE_NOT_IN_PAGE_FLAGS))
> 		ASSERT_EXCLUSIVE_BITS(folio->flags,
> 			      NODES_MASK << NODES_PGSHIFT);
> 
> But not sure if it's that much better.

It's still making an assumption of what the memdesc function we're calling will do.

-- 
Cheers,

David