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SubjectRe: [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning
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On 2/3/22 18:11, Andrew Morton wrote:
> On Tue, 1 Feb 2022 19:35:50 -0500 Waiman Long <longman@redhat.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

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