On 2/3/22 18:11, Andrew Morton wrote:
On Tue, 1 Feb 2022 19:35:50 -0500 Waiman Long <longman@xxxxxxxxxx> 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
