linux-kernel - Re: [PATCH] slub: fix __kmem_cache_empty for !CONFIG_SLUB

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Date:   Tue, 19 Jun 2018 17:49:44 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Shakeel Butt <shakeelb@...gle.com>
cc:     "Jason A . Donenfeld" <Jason@...c4.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Christoph Lameter <cl@...ux.com>,
        Pekka Enberg <penberg@...nel.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable@...r.kernel.org
Subject: Re: [PATCH] slub: fix __kmem_cache_empty for !CONFIG_SLUB_DEBUG

On Tue, 19 Jun 2018, Shakeel Butt wrote:

> diff --git a/mm/slub.c b/mm/slub.c
> index a3b8467c14af..731c02b371ae 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -3673,9 +3673,23 @@ static void free_partial(struct kmem_cache *s, struct kmem_cache_node *n)
>  
>  bool __kmem_cache_empty(struct kmem_cache *s)
>  {
> -	int node;
> +	int cpu, node;

Nit: wouldn't cpu be unused if CONFIG_SLUB_DEBUG is disabled?

>  	struct kmem_cache_node *n;
>  
> +	/*
> +	 * slabs_node will always be 0 for !CONFIG_SLUB_DEBUG. So, manually
> +	 * check slabs for all cpus.
> +	 */
> +	if (!IS_ENABLED(CONFIG_SLUB_DEBUG)) {
> +		for_each_online_cpu(cpu) {
> +			struct kmem_cache_cpu *c;
> +
> +			c = per_cpu_ptr(s->cpu_slab, cpu);
> +			if (c->page || slub_percpu_partial(c))
> +				return false;
> +		}
> +	}
> +
>  	for_each_kmem_cache_node(s, node, n)
>  		if (n->nr_partial || slabs_node(s, node))
>  			return false;

Wouldn't it just be better to allow {inc,dec}_slabs_node() to adjust the 
nr_slabs counter instead of doing the per-cpu iteration on every shutdown?