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

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Date:   Tue, 18 May 2021 14:35:07 +0800
From:   Muchun Song <songmuchun@...edance.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>, Michal Hocko <mhocko@...e.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>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] [PATCH] memcontrol: use flexible-array member

On Tue, May 18, 2021 at 1:20 PM wenhuizhang <wenhui@...ail.gwu.edu> 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
>
>
> 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[];

Have you tested this?

The compiler will complain about this. The output will be something like below.

    error: flexible array member in otherwise empty struct

Thanks.

>  } ____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
>