linux-kernel - Re: [PATCH] mm: slab: Avoid the use of one-element array and use struct

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Date:   Wed, 29 Jul 2020 19:12:31 -0500
From:   "Gustavo A. R. Silva" <gustavo@...eddedor.com>
To:     Qianli Zhao <zhaoqianligood@...il.com>, cl@...ux.com,
        penberg@...nel.org, rientjes@...gle.com, iamjoonsoo.kim@....com,
        akpm@...ux-foundation.org
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        clang-built-linux@...glegroups.com, zhaoqianli@...omi.com
Subject: Re: [PATCH] mm: slab: Avoid the use of one-element array and use
 struct_size() helper

Hi,

If you are going to copy/paste this, please at least CC the people
that is originally working on these changes, in this case _me_. One
needs to be very careful when doing these transformations. This
code doesn't even exist in linux-next.

If you want to land your first kernel patch, I encourage you to start
in drivers/staging/, while you get the hang of the kernel development
process and you know how to work with linux-next. :)

Thanks
--
Gustavo

On 7/29/20 09:50, Qianli Zhao wrote:
> From: Qianli Zhao <zhaoqianli@...omi.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@...omi.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[];
>  };
>  
>  /*
> 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;
>  
>