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);
_
