Re: [PATCH] mm/list_lru: make the case where mlru is NULL as unlikely

[Shakeel Butt <shakeel.butt@linux.dev>]

On Tue, Feb 25, 2025 at 11:30:20PM +0800, Jingxiang Zeng wrote:
> From: Zeng Jingxiang <linuszeng@tencent.com>
> 
> In the following memcg_list_lru_alloc() function, mlru here is almost
> always NULL, so in most cases this should save a function call, mark
> mlru as unlikely to optimize the code.
>         do {
>                 xas_lock_irqsave(&xas, flags);
>                 if (!xas_load(&xas) && !css_is_dying(&pos->css)) {
>                         xas_store(&xas, mlru);
>                         if (!xas_error(&xas))
>                                 mlru = NULL;
>                 }
>                 xas_unlock_irqrestore(&xas, flags);
>         } while (xas_nomem(&xas, GFP_KERNEL));
> >       if (mlru)
>                 kfree(mlru);
> 
> Reported-by: kernel test robot <lkp@intel.com>
> Closes: https://lore.kernel.org/oe-kbuild-all/202412290924.UTP7GH2Z-lkp@intel.com/
> Signed-off-by: Zeng Jingxiang <linuszeng@tencent.com>
> ---
>  mm/list_lru.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/mm/list_lru.c b/mm/list_lru.c
> index 064d2018e265..e7e13513ff8e 100644
> --- a/mm/list_lru.c
> +++ b/mm/list_lru.c
> @@ -552,7 +552,7 @@ static int memcg_list_lru_alloc(struct mem_cgroup *memcg, struct list_lru *lru)
>  			}
>  			xas_unlock_irqrestore(&xas, flags);
>  		} while (xas_nomem(&xas, GFP_KERNEL));
> -		if (mlru)
> +		if (unlikely(mlru))
>  			kfree(mlru);

The report is saying not to check at all. So, just remove the check and
simply call kfree(mlru) as it handles the NULL check efficiently.

>  		set_active_memcg(cur);
>  	} while (pos != memcg && !css_is_dying(&pos->css));
> -- 
> 2.43.5
> 
>