linux-kernel - Re: [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning

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Date:   Thu, 3 Feb 2022 22:29:23 -0500
From:   Waiman Long <longman@...hat.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     "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 v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc
 12 warning


On 2/3/22 18:11, Andrew Morton wrote:
> On Tue,  1 Feb 2022 19:35:50 -0500 Waiman Long <longman@...hat.com> wrote:
>
>> The gcc 12 compiler reports a "'mem_section' will never be NULL"
>> 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
>>          if (!mem_section[SECTION_NR_TO_ROOT(nr)])
>>                  return NULL;
>>         :
>>
>> It happens with both CONFIG_SPARSEMEM_EXTREME on and off. The mem_section
>> definition is
>>
>>      #ifdef CONFIG_SPARSEMEM_EXTREME
>>      extern struct mem_section **mem_section;
>>      #else
>>      extern struct mem_section mem_section[NR_SECTION_ROOTS][SECTIONS_PER_ROOT];
>>      #endif
>>
>> In the CONFIG_SPARSEMEM_EXTREME case, mem_section obviously cannot
>> be NULL, but *mem_section can be if memory hasn't been allocated for
>> the dynamic mem_section[] array yet. In the !CONFIG_SPARSEMEM_EXTREME
>> case, mem_section is a static 2-dimensional array and so the check
>> "!mem_section[SECTION_NR_TO_ROOT(nr)]" doesn't make sense.
>>
>> Fix this warning by checking for "!*mem_section" instead of
>> "!mem_section" and moving the "!mem_section[SECTION_NR_TO_ROOT(nr)]"
>> check up inside the CONFIG_SPARSEMEM_EXTREME block.
>>
>> ...
>>
>> --- a/include/linux/mmzone.h
>> +++ b/include/linux/mmzone.h
>> @@ -1390,11 +1390,9 @@ 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 || !mem_section[SECTION_NR_TO_ROOT(nr)])
>>   		return NULL;
>>   #endif
>> -	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
>> -		return NULL;
>>   	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
>>   }
>>   extern size_t mem_section_usage_size(void);
> What does the v1->v2 change do?
>
> --- a/include/linux/mmzone.h~mm-sparsemem-fix-mem_section-will-never-be-null-gcc-12-warning-v2
> +++ a/include/linux/mmzone.h
> @@ -1390,11 +1390,9 @@ static inline unsigned long *section_to_
>   static inline struct mem_section *__nr_to_section(unsigned long nr)
>   {
>   #ifdef CONFIG_SPARSEMEM_EXTREME
> -	if (!*mem_section)
> +	if (!*mem_section || !mem_section[SECTION_NR_TO_ROOT(nr)])
>   		return NULL;
>   #endif
> -	if (!mem_section[SECTION_NR_TO_ROOT(nr)])
> -		return NULL;
>   	return &mem_section[SECTION_NR_TO_ROOT(nr)][nr & SECTION_ROOT_MASK];
>   }
>   extern size_t mem_section_usage_size(void);
> _

When !CONFIG_SPARSEMEM_EXTREME, mem_section is really a static 2-D 
array. Since it is not a table of pointers, 
mem_section[SECTION_NR_TO_ROOT(nr)] has no real meaning. That is why the 
compiler is complaining. This check isn't applicable in the 
!CONFIG_SPARSEMEM_EXTREME case, but it is meaningful for 
CONFIG_SPARSEMEM_EXTREME. That is why it is pulled into the 
CONFIG_SPARSEMEM_EXTREME block.

Thanks,
Longman