Re: [PATCH v3] mm: annotate data-race in cpu_needs_drain()

[Lorenzo Stoakes <ljs@kernel.org>]

On Fri, Jun 26, 2026 at 01:37:00PM +0800, Xuewen Wang wrote:
> KCSAN reports a data-race when cpu_needs_drain() reads another CPU's
> per-cpu folio_batch->nr without locking, while the owning CPU writes
> to it via folio_batch_add().
>
> Reading a slightly stale value is harmless -- cpu_needs_drain() only
> decides whether to schedule a drain, and the next iteration of
> __lru_add_drain_all() will re-check. Use data_race() to annotate
> the intentional race.
>
> Signed-off-by: Xuewen Wang <wangxuewen@kylinos.cn>

LGTM, so:

Reviewed-by: Lorenzo Stoakes <ljs@kernel.org>

> ---
> Changes in v3:
> - Wrap the entire || expression in a single data_race() instead of wrapping
>   each folio_batch_count() call individually, as suggested by Pedro and Lorenzo.
>   This is equally effective and more readable.
> - Remove data_race() from need_mlock_drain(), as it is now covered by the data_race()
>   in cpu_needs_drain().
> v2:
>   https://lore.kernel.org/all/20260625065153.1581419-1-wangxuewen@kylinos.cn/
> v1:
>   https://lore.kernel.org/all/20260624092606.1083449-1-wangxuewen@kylinos.cn/
> ---
>  mm/swap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/mm/swap.c b/mm/swap.c
> index 588f50d8f1a8..46ea207e0624 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -828,13 +828,13 @@ static bool cpu_needs_drain(unsigned int cpu)
>  	struct cpu_fbatches *fbatches = &per_cpu(cpu_fbatches, cpu);
>
>  	/* Check these in order of likelihood that they're not zero */
> -	return folio_batch_count(&fbatches->lru_add) ||
> +	return data_race(folio_batch_count(&fbatches->lru_add) ||
>  		folio_batch_count(&fbatches->lru_move_tail) ||
>  		folio_batch_count(&fbatches->lru_deactivate_file) ||
>  		folio_batch_count(&fbatches->lru_deactivate) ||
>  		folio_batch_count(&fbatches->lru_lazyfree) ||
>  		folio_batch_count(&fbatches->lru_activate) ||
> -		need_mlock_drain(cpu) ||
> +		need_mlock_drain(cpu)) ||
>  		has_bh_in_lru(cpu, NULL);
>  }
>
> --
> 2.25.1
>

Cheers, Lorenzo