linux-kernel - Re: [PATCH] mm/slub: remove useless kmem_cache

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Date:   Mon, 10 Aug 2020 12:44:52 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Abel Wu <wuyun.wu@...wei.com>
cc:     Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        liu.xiang6@....com.cn,
        "open list:SLAB ALLOCATOR" <linux-mm@...ck.org>,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm/slub: remove useless kmem_cache_debug

On Mon, 10 Aug 2020, wuyun.wu@...wei.com wrote:

> From: Abel Wu <wuyun.wu@...wei.com>
> 
> The commit below is incomplete, as it didn't handle the add_full() part.
> commit a4d3f8916c65 ("slub: remove useless kmem_cache_debug() before remove_full()")
> 
> Signed-off-by: Abel Wu <wuyun.wu@...wei.com>
> ---
>  mm/slub.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index fe81773..0b021b7 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2182,7 +2182,8 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>  		}
>  	} else {
>  		m = M_FULL;
> -		if (kmem_cache_debug(s) && !lock) {
> +#ifdef CONFIG_SLUB_DEBUG
> +		if (!lock) {
>  			lock = 1;
>  			/*
>  			 * This also ensures that the scanning of full
> @@ -2191,6 +2192,7 @@ static void deactivate_slab(struct kmem_cache *s, struct page *page,
>  			 */
>  			spin_lock(&n->list_lock);
>  		}
> +#endif
>  	}
> 
>  	if (l != m) {

This should be functionally safe, I'm wonder if it would make sense to 
only check for SLAB_STORE_USER here instead of kmem_cache_debug(), 
however, since that should be the only context in which we need the 
list_lock for add_full()?  It seems more explicit.