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

[Waiman Long]

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