Re: [PATCH] mm/swap.c: workaround for_each_cpu() bug on UP kernel.

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2019/02/12 19:11, Michal Hocko wrote:
> This patch is ugly as hell! I do agree that for_each_cpu not working on
> CONFIG_SMP=n sucks but why do we even care about lru_add_drain_all when
> there is a single cpu? Why don't we simply do
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index aa483719922e..952f24b09070 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -660,6 +660,7 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
>  
>  static DEFINE_PER_CPU(struct work_struct, lru_add_drain_work);
>  
> +#ifdef CONFIG_SMP
>  /*
>   * Doesn't need any cpu hotplug locking because we do rely on per-cpu
>   * kworkers being shut down before our page_alloc_cpu_dead callback is
> @@ -702,6 +703,10 @@ void lru_add_drain_all(void)
>  
>  	mutex_unlock(&lock);
>  }
> +#else
> +#define lru_add_drain_all() lru_add_drain()
> +
> +#endif

If there is no need to evaluate the "if" conditions, I'm fine with this shortcut.