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

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Date:   Wed, 2 Feb 2022 08:07:55 +0200
From:   Mike Rapoport <rppt@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ingo Molnar <mingo@...nel.org>, 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 Tue, Feb 01, 2022 at 02:29:24PM -0500, 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.

Hmm, if mem_section cannot be NULL, it means if will be never allocated,
no?

static void __init memory_present(int nid, unsigned long start, unsigned long end)
{
	unsigned long pfn;

#ifdef CONFIG_SPARSEMEM_EXTREME
	if (unlikely(!mem_section)) {
		unsigned long size, align;

		size = sizeof(struct mem_section *) * NR_SECTION_ROOTS;
		align = 1 << (INTERNODE_CACHE_SHIFT);
		mem_section = memblock_alloc(size, align);
		if (!mem_section)
			panic("%s: Failed to allocate %lu bytes align=0x%lx\n",
			      __func__, size, align);
	}
#endif

...

> 
> Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
> 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)])
> -- 
> 2.27.0
> 
> 

-- 
Sincerely yours,
Mike.