linux-kernel - Re: [PATCH] memcontrol: use flexible-array member

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Message-ID: <YKN19XI/6Wpk9+Eh@dhcp22.suse.cz>
Date:   Tue, 18 May 2021 10:08:21 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     wenhuizhang <wenhui@...ail.gwu.edu>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>,
        Muchun Song <songmuchun@...edance.com>,
        Yang Shi <shy828301@...il.com>, Alex Shi <alexs@...nel.org>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        Wei Yang <richard.weiyang@...il.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] memcontrol: use flexible-array member

On Tue 18-05-21 01:20:36, wenhuizhang wrote:
> 
> Change depracated zero-length-and-one-element-arrays into flexible 
> array member.
> Zero-length and one-element arrays detected by Lukas's CodeChecker.
> Zero/one element arrays causes undefined behaviours if sizeof() used.
> 
> https://www.kernel.org/doc/html/latest/processd/deprecated.html

I do understand why zero-length structures are discouraged for trail
space but does the same concern apply to internal padding inside
structures to enforce alignment to different cache lines? AFAIK this is
a trick we use at other places and I do not remember having heard about
any problems.

nodeinfo is a different case where flexible arrays seems reasonable to
me.

> Signed-off-by: wenhuizhang<wenhui@...ail.gwu.edu>
> ---
>  include/linux/memcontrol.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
> index 0ce97eff79e2..f879efb1bc6e 100644
> --- a/include/linux/memcontrol.h
> +++ b/include/linux/memcontrol.h
> @@ -190,7 +190,7 @@ enum memcg_kmem_state {
>  
>  #if defined(CONFIG_SMP)
>  struct memcg_padding {
> -	char x[0];
> +	char x[];
>  } ____cacheline_internodealigned_in_smp;
>  #define MEMCG_PADDING(name)      struct memcg_padding name;
>  #else
> @@ -349,7 +349,7 @@ struct mem_cgroup {
>  	struct deferred_split deferred_split_queue;
>  #endif
>  
> -	struct mem_cgroup_per_node *nodeinfo[0];
> +	struct mem_cgroup_per_node *nodeinfo[];
>  	/* WARNING: nodeinfo must be the last member here */
>  };
>  
> -- 
> 2.17.1

-- 
Michal Hocko
SUSE Labs