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Date: Wed, 24 Jun 2026 14:01:04 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Hui Zhu <hui.zhu@linux.dev>
Cc: David Hildenbrand <david@kernel.org>, Lorenzo Stoakes <ljs@kernel.org>,
 "Liam R. Howlett" <liam@infradead.org>, Vlastimil Babka
 <vbabka@kernel.org>, Mike Rapoport <rppt@kernel.org>, Suren Baghdasaryan
 <surenb@google.com>, Michal Hocko <mhocko@suse.com>, linux-mm@kvack.org,
 linux-kernel@vger.kernel.org, Hui Zhu <zhuhui@kylinos.cn>
Subject: Re: [PATCH v2] mm: avoid KCSAN false positive in memdesc_nid()
Message-Id: <20260624140104.eacc15e291eec123bc7b3349@linux-foundation.org>
In-Reply-To: <20260623084432.701120-1-hui.zhu@linux.dev>
References: <20260623084432.701120-1-hui.zhu@linux.dev>
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On Tue, 23 Jun 2026 16:44:32 +0800 Hui Zhu <hui.zhu@linux.dev> wrote:

> From: Hui Zhu <zhuhui@kylinos.cn>
> 
> KCSAN reports a data race between page_to_nid()/folio_nid() 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 occupies a fixed bit-range of page->flags that is set
> once at page init and never modified afterwards, so it can never
> overlap with the low PG_locked/PG_waiters bits touched by the folio
> lock path.
> 
> Use ASSERT_EXCLUSIVE_BITS() in memdesc_nid() to scope the exemption
> to just the node-id bits, consistent with how memdesc_zonenum()
> already handles the same class of race for the zone-id bits.
> 
> ...
>
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -2290,6 +2290,7 @@ int memdesc_nid(memdesc_flags_t mdf);
>  #else
>  static inline int memdesc_nid(memdesc_flags_t mdf)
>  {
> +	ASSERT_EXCLUSIVE_BITS(mdf.f, NODES_MASK << NODES_PGSHIFT);
>  	return (mdf.f >> NODES_PGSHIFT) & NODES_MASK;
>  }
>  #endif

It seems weird to be doing this against a local variable within a
random function, seemingly unrelated to the problematic functions which
you've identified.

Seems that it fooled Sashiko:
	https://sashiko.dev/#/patchset/20260623084432.701120-1-hui.zhu@linux.dev

I'm wondering what the heck is going on here?