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

[Lorenzo Stoakes]

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.

(There's precedent for that form of it in mm/numa_memblks.c)

>
> --
> Cheers,
>
> David

Thanks, Lorenzo