* [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning
@ 2022-02-02 0:35 Waiman Long
2022-02-03 23:11 ` Andrew Morton
0 siblings, 1 reply; 3+ messages in thread
From: Waiman Long @ 2022-02-02 0:35 UTC (permalink / raw)
To: Andrew Morton, Kirill A. Shutemov, Ingo Molnar
Cc: linux-mm, linux-kernel, Justin Forbes, Rafael Aquini, 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
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning
2022-02-02 0:35 [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning Waiman Long
@ 2022-02-03 23:11 ` Andrew Morton
2022-02-04 3:29 ` Waiman Long
0 siblings, 1 reply; 3+ messages in thread
From: Andrew Morton @ 2022-02-03 23:11 UTC (permalink / raw)
To: Waiman Long
Cc: Kirill A. Shutemov, Ingo Molnar, linux-mm, linux-kernel,
Justin Forbes, Rafael Aquini
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);
_
^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning
2022-02-03 23:11 ` Andrew Morton
@ 2022-02-04 3:29 ` Waiman Long
0 siblings, 0 replies; 3+ messages in thread
From: Waiman Long @ 2022-02-04 3:29 UTC (permalink / raw)
To: Andrew Morton
Cc: Kirill A. Shutemov, Ingo Molnar, linux-mm, linux-kernel,
Justin Forbes, Rafael Aquini
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
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2022-02-04 3:29 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-02-02 0:35 [PATCH v2] mm/sparsemem: Fix 'mem_section' will never be NULL gcc 12 warning Waiman Long
2022-02-03 23:11 ` Andrew Morton
2022-02-04 3:29 ` Waiman Long
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox