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Date:   Wed, 29 Jul 2020 16:40:42 -0700 (PDT)
From:   David Rientjes <rientjes@google.com>
To:     Qianli Zhao <zhaoqianligood@gmail.com>,
        Roman Gushchin <guro@fb.com>
cc:     cl@linux.com, penberg@kernel.org, iamjoonsoo.kim@lge.com,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
        zhaoqianli@xiaomi.com
Subject: Re: [PATCH] mm: slab: Avoid the use of one-element array and use
 struct_size() helper
In-Reply-To: <1596034214-15010-1-git-send-email-zhaoqianligood@gmail.com>
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References: <1596034214-15010-1-git-send-email-zhaoqianligood@gmail.com>
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On Wed, 29 Jul 2020, Qianli Zhao wrote:

> From: Qianli Zhao <zhaoqianli@xiaomi.com>
> 
> There is a regular need in the kernel to provide a way to declare having a
> dynamically sized set of trailing elements in a structure. Kernel code should
> always use “flexible array members”[1] for these cases. The older style of
> one-element or zero-length arrays should no longer be used[2].
> 
> Make use of the struct_size() helper instead of an open-coded version
> in order to avoid any potential type mistakes.
> 
> [1] https://en.wikipedia.org/wiki/Flexible_array_member
> [2] https://github.com/KSPP/linux/issues/21
> 
> Signed-off-by: Qianli Zhao <zhaoqianli@xiaomi.com>
> ---
>  mm/slab.h        | 2 +-
>  mm/slab_common.c | 7 ++-----
>  2 files changed, 3 insertions(+), 6 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 74f7e09..c12fb65 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -34,7 +34,7 @@ struct kmem_cache {
>  
>  struct memcg_cache_array {
>  	struct rcu_head rcu;
> -	struct kmem_cache *entries[0];
> +	struct kmem_cache *entries[];
>  };
>  
>  /*

This is removed in the -mm tree, see 
https://www.ozlabs.org/~akpm/mmotm/broken-out/mm-memcg-slab-use-a-single-set-of-kmem_caches-for-all-allocations.patch

> diff --git a/mm/slab_common.c b/mm/slab_common.c
> index fe8b684..56f4818 100644
> --- a/mm/slab_common.c
> +++ b/mm/slab_common.c
> @@ -166,9 +166,7 @@ static int init_memcg_params(struct kmem_cache *s,
>  	if (!memcg_nr_cache_ids)
>  		return 0;
>  
> -	arr = kvzalloc(sizeof(struct memcg_cache_array) +
> -		       memcg_nr_cache_ids * sizeof(void *),
> -		       GFP_KERNEL);
> +	arr = kvzalloc(struct_size(arr, entries, memcg_nr_cache_ids), GFP_KERNEL);
>  	if (!arr)
>  		return -ENOMEM;
>  
> @@ -199,8 +197,7 @@ static int update_memcg_params(struct kmem_cache *s, int new_array_size)
>  {
>  	struct memcg_cache_array *old, *new;
>  
> -	new = kvzalloc(sizeof(struct memcg_cache_array) +
> -		       new_array_size * sizeof(void *), GFP_KERNEL);
> +	new = kvzalloc(struct_size(new, entries, new_array_size), GFP_KERNEL);
>  	if (!new)
>  		return -ENOMEM;
>  
> -- 
> 2.7.4
> 
> 
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