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

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

[Waiman Long]

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.

Fixes: 83e3c48729d9 ("mm/sparsemem: Allocate mem_section at runtime for CONFIG_SPARSEMEM_EXTREME=y")
Fixes: 3e347261a80b ("sparsemem extreme implementation")
Reported-by: Justin Forbes <jforbes@redhat.com>
Signed-off-by: Waiman Long <longman@redhat.com>
---
 include/linux/mmzone.h | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h
index aed44e9b5d89..08517376c765 100644
--- 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);
-- 
2.27.0

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

[Andrew Morton]

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);
_

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@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