linux-kernel - Re: [PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

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Date:   Tue, 1 Feb 2022 14:34:02 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Justin Forbes <jforbes@...hat.com>,
        Rafael Aquini <aquini@...hat.com>
Subject: Re: [PATCH] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12
 warning

On 2/1/22 14:29, Waiman Long wrote:
> The gcc 12 compiler reports a warning on the following code:
>
>      static inline struct mem_section *__nr_to_section(unsigned long nr)
>      {
>      #ifdef CONFIG_SPARSEMEM_EXTREME
>          if (!mem_section)
>                  return NULL;
>      #endif
>         :
>
> With CONFIG_SPARSEMEM_EXTREME on, the mem_section definition is
>
>      extern struct mem_section **mem_section;
>
> Obviously, mem_section cannot be NULL, but *mem_section can be if memory
> hasn't been allocated for the dynamic mem_section[] array yet. Fix this
> warning by checking for "!*mem_section" instead.
>
> Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")

Sorry, I forgot to add the line

Reported-by: Justin Forbes <jforbes@...hat.com>

> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
>   include/linux/mmzone.h | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
> index aed44e9b5d89..bd1b19925f3b 100644
> --- a/include/linux/mmzone.h
> +++ b/include/linux/mmzone.h
> @@ -1390,7 +1390,7 @@ static inline unsigned long *section_to_usemap(struct mem_section *ms)
>   static inline struct mem_section *__nr_to_section(unsigned long nr)
>   {
>   #ifdef CONFIG_SPARSEMEM_EXTREME
> -	if (!mem_section)
> +	if (!*mem_section)
>   		return NULL;
>   #endif
>   	if (!mem_section[SECTION_NR_TO_ROOT(nr)])