[PATCH 1/5] mm/Kconfig: add ARCH_NEEDS_WEAK_PER_CPU option

[PATCH 1/5] mm/Kconfig: add ARCH_NEEDS_WEAK_PER_CPU option

[Hao Ge]

From: Hao Ge <gehao@kylinos.cn>

Recently discovered this entry while checking kallsyms on ARM64:
ffff800083e509c0 D _shared_alloc_tag

If ARCH_NEEDS_WEAK_PER_CPU is not defined((it is only defined for
s390 and alpha architectures),there's no need to statically define
the percpu variable _shared_alloc_tag. As the number of CPUs
increases,the wasted memory will grow correspondingly.

Therefore,we need to implement isolation for this purpose.

However,currently ARCH_NEEDS_WEAK_PER_CPU is a #define and
is enclosed within the #if defined(MODULE) conditional block.

When building the core kernel code for s390 or alpha architectures,
ARCH_NEEDS_WEAK_PER_CPU remains undefined (as it is gated by #if defined(MODULE)).
However,when building modules for these architectures,the macro is explicitly defined.

Therefore,we need to make ARCH_NEEDS_WEAK_PER_CPU a Kconfig option.
And replace all instances of ARCH_NEEDS_WEAK_PER_CPU in the kernel
code with MODULE_NEEDS_WEAK_PER_CPU,gated by #ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU.
Then,when defining the percpu variable _shared_alloc_tag,wrap it
with the CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.

Therefore,we should add the ARCH_NEEDS_WEAK_PER_CPU option
to the mm Kconfig file.

Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 mm/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index e113f713b493..36bb4d6fd360 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -929,6 +929,10 @@ config ARCH_SUPPORTS_PUD_PFNMAP
 	def_bool y
 	depends on ARCH_SUPPORTS_HUGE_PFNMAP && HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 
+# s390 and alpha be enabled,see comments for DECLARE_PER_CPU_SECTION
+config ARCH_NEEDS_WEAK_PER_CPU
+       bool
+
 #
 # UP and nommu archs use km based percpu allocator
 #
-- 
2.25.1

[PATCH 0/5] mm: Restrict the static definition of the per-CPU variable _shared_alloc_tag to s390 and alpha architectures only

[Hao Ge]

From: Hao Ge <gehao@kylinos.cn>

Recently discovered this entry while checking kallsyms on ARM64:
ffff800083e509c0 D _shared_alloc_tag

If ARCH_NEEDS_WEAK_PER_CPU is not defined((it is only defined for
s390 and alpha architectures),there's no need to statically define
the percpu variable _shared_alloc_tag. As the number of CPUs
increases,the wasted memory will grow correspondingly.

Therefore,we need to implement isolation for this purpose.

However,currently ARCH_NEEDS_WEAK_PER_CPU is a #define and
is enclosed within the #if defined(MODULE) conditional block.

When building the core kernel code for s390 or alpha architectures,
ARCH_NEEDS_WEAK_PER_CPU remains undefined (as it is gated
by #if defined(MODULE)). However,when building modules for these
architectures,the macro is explicitly defined.

Therefore,we need to make ARCH_NEEDS_WEAK_PER_CPU a Kconfig option.
And replace all instances of ARCH_NEEDS_WEAK_PER_CPU in the kernel
code with MODULE_NEEDS_WEAK_PER_CPU,gated
by#ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU. Then,when defining the percpu
variable _shared_alloc_tag,wrap it with the
CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.

The following version could be regarded as Version 1:
https://lore.kernel.org/all/20250529073537.563107-1-hao.ge@linux.dev/
But unfortunately,it caused build errors on s390.
Based on Suren's guidance and suggestions,
I've refined it into this patch series.
Many thanks to Suren for his patient instruction.

Verify:
     1. On Arm64:
        nm vmlinux | grep "_shared_alloc_tag",no output is returned.
     2. On S390:
        Compile tested.
        nm vmlinux | grep "_shared_alloc_tag"
        00000000015605b4 r __crc__shared_alloc_tag
        0000000001585fef r __kstrtab__shared_alloc_tag
        0000000001586897 r __kstrtabns__shared_alloc_tag
        00000000014f6548 r __ksymtab__shared_alloc_tag
        0000000001a8fa28 D _shared_alloc_tag
        nm net/ceph/libceph.ko | grep "_shared"
        U _shared_alloc_tag
     3. On alpha
        Compile tested.
        nm vmlinux | grep "_shared_alloc_tag"
        fffffc0000b080fa r __kstrtab__shared_alloc_tag
        fffffc0000b07ee7 r __kstrtabns__shared_alloc_tag
        fffffc0000adee98 r __ksymtab__shared_alloc_tag
        fffffc0000b83d38 D _shared_alloc_tag
        nm crypto/cryptomgr.ko | grep "_share"
        U _shared_alloc_tag

Hao Ge (5):
  mm/Kconfig: add ARCH_NEEDS_WEAK_PER_CPU option
  alpha: Modify the definition logic of WEAK_PER_CPU
  s390: Modify the definition logic of WEAK_PER_CPU
  mm: use MODULE_NEEDS_WEAK_PER_CPU instead of ARCH_NEEDS_WEAK_PER_CPU
  mm/alloc_tag: add the CONFIG_ARCH_NEEDS_WEAK_PER_CPU macro when
    statically defining the percpu variable _shared_alloc_tag

 arch/alpha/Kconfig              | 1 +
 arch/alpha/include/asm/percpu.h | 4 ++--
 arch/s390/Kconfig               | 1 +
 arch/s390/include/asm/percpu.h  | 4 ++--
 include/linux/alloc_tag.h       | 6 +++---
 include/linux/percpu-defs.h     | 4 ++--
 lib/alloc_tag.c                 | 2 ++
 mm/Kconfig                      | 4 ++++
 8 files changed, 17 insertions(+), 9 deletions(-)

-- 
2.25.1

[PATCH 1/5] mm/Kconfig: add ARCH_NEEDS_WEAK_PER_CPU option

[Hao Ge]

From: Hao Ge <gehao@kylinos.cn>

Recently discovered this entry while checking kallsyms on ARM64:
ffff800083e509c0 D _shared_alloc_tag

If ARCH_NEEDS_WEAK_PER_CPU is not defined((it is only defined for
s390 and alpha architectures),there's no need to statically define
the percpu variable _shared_alloc_tag. As the number of CPUs
increases,the wasted memory will grow correspondingly.

Therefore,we need to implement isolation for this purpose.

However,currently ARCH_NEEDS_WEAK_PER_CPU is a #define and
is enclosed within the #if defined(MODULE) conditional block.

When building the core kernel code for s390 or alpha architectures,
ARCH_NEEDS_WEAK_PER_CPU remains undefined (as it is gated
by #if defined(MODULE)).However,when building modules for these
architectures,the macro is explicitly defined.

Therefore,we need to make ARCH_NEEDS_WEAK_PER_CPU a Kconfig option.
And replace all instances of ARCH_NEEDS_WEAK_PER_CPU in the kernel
code with MODULE_NEEDS_WEAK_PER_CPU,gated
by #ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU.Then,when defining
the percpu variable _shared_alloc_tag,wrap it
with the CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.

Therefore,we should add the ARCH_NEEDS_WEAK_PER_CPU option
to the mm Kconfig file.

Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 mm/Kconfig | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/mm/Kconfig b/mm/Kconfig
index e113f713b493..36bb4d6fd360 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -929,6 +929,10 @@ config ARCH_SUPPORTS_PUD_PFNMAP
 	def_bool y
 	depends on ARCH_SUPPORTS_HUGE_PFNMAP && HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 
+# s390 and alpha be enabled,see comments for DECLARE_PER_CPU_SECTION
+config ARCH_NEEDS_WEAK_PER_CPU
+       bool
+
 #
 # UP and nommu archs use km based percpu allocator
 #
-- 
2.25.1

[PATCH 2/5] alpha: Modify the definition logic of WEAK_PER_CPU

[Hao Ge]

From: Hao Ge <gehao@kylinos.cn>

As stated in the first patch of this patch series,
we make ARCH_NEEDS_WEAK_PER_CPU a Kconfig option,
and replace all instances of ARCH_NEEDS_WEAK_PER_CPU
in the kernel code with MODULE_NEEDS_WEAK_PER_CPU,
gated by #ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU.

We'll make corresponding changes for the alpha
architecture.

Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 arch/alpha/Kconfig              | 1 +
 arch/alpha/include/asm/percpu.h | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 109a4cddcd13..ce9bc842e384 100644
--- a/arch/alpha/Kconfig
+++ b/arch/alpha/Kconfig
@@ -7,6 +7,7 @@ config ALPHA
 	select ARCH_HAS_DMA_OPS if PCI
 	select ARCH_MIGHT_HAVE_PC_PARPORT
 	select ARCH_MIGHT_HAVE_PC_SERIO
+	select ARCH_NEEDS_WEAK_PER_CPU
 	select ARCH_NO_PREEMPT
 	select ARCH_NO_SG_CHAIN
 	select ARCH_USE_CMPXCHG_LOCKREF
diff --git a/arch/alpha/include/asm/percpu.h b/arch/alpha/include/asm/percpu.h
index 6923249f2d49..5d8b75ec3bf2 100644
--- a/arch/alpha/include/asm/percpu.h
+++ b/arch/alpha/include/asm/percpu.h
@@ -10,8 +10,8 @@
  *
  * Always use weak definitions for percpu variables in modules.
  */
-#if defined(MODULE) && defined(CONFIG_SMP)
-#define ARCH_NEEDS_WEAK_PER_CPU
+#if defined(MODULE) && defined(CONFIG_SMP) && defined(CONFIG_ARCH_NEEDS_WEAK_PER_CPU)
+#define MODULE_NEEDS_WEAK_PER_CPU
 #endif
 
 #include <asm-generic/percpu.h>
-- 
2.25.1

[PATCH 3/5] s390: Modify the definition logic of WEAK_PER_CPU

[Hao Ge]

From: Hao Ge <gehao@kylinos.cn>

As stated in the first patch of this patch series,
we make ARCH_NEEDS_WEAK_PER_CPU a Kconfig option,
and replace all instances of ARCH_NEEDS_WEAK_PER_CPU
in the kernel code with MODULE_NEEDS_WEAK_PER_CPU,
gated by #ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU.

We'll make corresponding changes for the s390
architecture.

Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 arch/s390/Kconfig              | 1 +
 arch/s390/include/asm/percpu.h | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 0c16dc443e2f..716031d22346 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -132,6 +132,7 @@ config S390
 	select ARCH_INLINE_WRITE_UNLOCK_IRQ
 	select ARCH_INLINE_WRITE_UNLOCK_IRQRESTORE
 	select ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
+	select ARCH_NEEDS_WEAK_PER_CPU
 	select ARCH_STACKWALK
 	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_SUPPORTS_DEBUG_PAGEALLOC
diff --git a/arch/s390/include/asm/percpu.h b/arch/s390/include/asm/percpu.h
index 84f6b8357b45..a202baef86fb 100644
--- a/arch/s390/include/asm/percpu.h
+++ b/arch/s390/include/asm/percpu.h
@@ -17,8 +17,8 @@
  * per cpu area, use weak definitions to force the compiler to
  * generate external references.
  */
-#if defined(MODULE)
-#define ARCH_NEEDS_WEAK_PER_CPU
+#if defined(MODULE) && defined(CONFIG_ARCH_NEEDS_WEAK_PER_CPU)
+#define MODULE_NEEDS_WEAK_PER_CPU
 #endif
 
 /*
-- 
2.25.1

[PATCH 4/5] mm: use MODULE_NEEDS_WEAK_PER_CPU instead of ARCH_NEEDS_WEAK_PER_CPU

[Hao Ge]

From: Hao Ge <gehao@kylinos.cn>

We have made ARCH_NEEDS_WEAK_PER_CPU a Kconfig option and
introduced MODULE_NEEDS_WEAK_PER_CPU,so we should use
MODULE_NEEDS_WEAK_PER_CPU instead of ARCH_NEEDS_WEAK_PER_CPU
in all current places within the mm/ module.

Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 include/linux/alloc_tag.h   | 6 +++---
 include/linux/percpu-defs.h | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
index 8f7931eb7d16..f349cca0ebed 100644
--- a/include/linux/alloc_tag.h
+++ b/include/linux/alloc_tag.h
@@ -88,7 +88,7 @@ static inline struct alloc_tag *ct_to_alloc_tag(struct codetag *ct)
 	return container_of(ct, struct alloc_tag, ct);
 }
 
-#ifdef ARCH_NEEDS_WEAK_PER_CPU
+#ifdef MODULE_NEEDS_WEAK_PER_CPU
 /*
  * When percpu variables are required to be defined as weak, static percpu
  * variables can't be used inside a function (see comments for DECLARE_PER_CPU_SECTION).
@@ -102,7 +102,7 @@ DECLARE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
 		.ct = CODE_TAG_INIT,						\
 		.counters = &_shared_alloc_tag };
 
-#else /* ARCH_NEEDS_WEAK_PER_CPU */
+#else /* MODULE_NEEDS_WEAK_PER_CPU */
 
 #ifdef MODULE
 
@@ -123,7 +123,7 @@ DECLARE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
 
 #endif /* MODULE */
 
-#endif /* ARCH_NEEDS_WEAK_PER_CPU */
+#endif /* MODULE_NEEDS_WEAK_PER_CPU */
 
 DECLARE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
 			mem_alloc_profiling_key);
diff --git a/include/linux/percpu-defs.h b/include/linux/percpu-defs.h
index 0aeb0e276a3e..b4ecfc3a7b2b 100644
--- a/include/linux/percpu-defs.h
+++ b/include/linux/percpu-defs.h
@@ -64,13 +64,13 @@
  * 2. Static percpu variables cannot be defined inside a function.
  *
  * Archs which need weak percpu definitions should define
- * ARCH_NEEDS_WEAK_PER_CPU in asm/percpu.h when necessary.
+ * MODULE_NEEDS_WEAK_PER_CPU in asm/percpu.h when necessary.
  *
  * To ensure that the generic code observes the above two
  * restrictions, if CONFIG_DEBUG_FORCE_WEAK_PER_CPU is set weak
  * definition is used for all cases.
  */
-#if defined(ARCH_NEEDS_WEAK_PER_CPU) || defined(CONFIG_DEBUG_FORCE_WEAK_PER_CPU)
+#if defined(MODULE_NEEDS_WEAK_PER_CPU) || defined(CONFIG_DEBUG_FORCE_WEAK_PER_CPU)
 /*
  * __pcpu_scope_* dummy variable is used to enforce scope.  It
  * receives the static modifier when it's used in front of
-- 
2.25.1

[PATCH 5/5] mm/alloc_tag: add the CONFIG_ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable _shared_alloc_tag

[Hao Ge]

From: Hao Ge <gehao@kylinos.cn>

Recently discovered this entry while checking kallsyms on ARM64:
ffff800083e509c0 D _shared_alloc_tag

If CONFIG_ARCH_NEEDS_WEAK_PER_CPU is not defined(it is only defined for
s390 and alpha architectures),there's no need to statically define
the percpu variable _shared_alloc_tag. As the number of CPUs
increases,the wasted memory will grow correspondingly.

Enclose the definition of _shared_alloc_tag within the
CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.

Suggested-by: Suren Baghdasaryan <surenb@google.com>
Signed-off-by: Hao Ge <gehao@kylinos.cn>
---
 lib/alloc_tag.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index c7f602fa7b23..14fd66f26e42 100644
--- a/lib/alloc_tag.c
+++ b/lib/alloc_tag.c
@@ -24,8 +24,10 @@ static bool mem_profiling_support;
 
 static struct codetag_type *alloc_tag_cttype;
 
+#ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU
 DEFINE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
 EXPORT_SYMBOL(_shared_alloc_tag);
+#endif
 
 DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
 			mem_alloc_profiling_key);
-- 
2.25.1

Re: [PATCH 5/5] mm/alloc_tag: add the CONFIG_ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable _shared_alloc_tag

[Mike Rapoport]

On Thu, Jun 12, 2025 at 04:27:30PM +0800, Hao Ge wrote:
> From: Hao Ge <gehao@kylinos.cn>
> 
> Recently discovered this entry while checking kallsyms on ARM64:
> ffff800083e509c0 D _shared_alloc_tag
> 
> If CONFIG_ARCH_NEEDS_WEAK_PER_CPU is not defined(it is only defined for
> s390 and alpha architectures),there's no need to statically define
> the percpu variable _shared_alloc_tag. As the number of CPUs
> increases,the wasted memory will grow correspondingly.
> 
> Enclose the definition of _shared_alloc_tag within the
> CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.
> 
> Suggested-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Hao Ge <gehao@kylinos.cn>
> ---
>  lib/alloc_tag.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> index c7f602fa7b23..14fd66f26e42 100644
> --- a/lib/alloc_tag.c
> +++ b/lib/alloc_tag.c
> @@ -24,8 +24,10 @@ static bool mem_profiling_support;
>  
>  static struct codetag_type *alloc_tag_cttype;
>  
> +#ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU

It should be enough to add #ifdef ARCH_NEEDS_WEAK_PER_CPU here instead of
all the churn.

>  DEFINE_PER_CPU(struct alloc_tag_counters, _shared_alloc_tag);
>  EXPORT_SYMBOL(_shared_alloc_tag);
> +#endif
>  
>  DEFINE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
>  			mem_alloc_profiling_key);
> -- 
> 2.25.1
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH 2/5] alpha: Modify the definition logic of WEAK_PER_CPU

[Heiko Carstens]

On Thu, Jun 12, 2025 at 04:27:27PM +0800, Hao Ge wrote:
> From: Hao Ge <gehao@kylinos.cn>
> 
> As stated in the first patch of this patch series,
> we make ARCH_NEEDS_WEAK_PER_CPU a Kconfig option,
> and replace all instances of ARCH_NEEDS_WEAK_PER_CPU
> in the kernel code with MODULE_NEEDS_WEAK_PER_CPU,
> gated by #ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU.
> 
> We'll make corresponding changes for the alpha
> architecture.
> 
> Suggested-by: Suren Baghdasaryan <surenb@google.com>
> Signed-off-by: Hao Ge <gehao@kylinos.cn>
> ---
>  arch/alpha/Kconfig              | 1 +
>  arch/alpha/include/asm/percpu.h | 4 ++--
>  2 files changed, 3 insertions(+), 2 deletions(-)

...

> +	select ARCH_NEEDS_WEAK_PER_CPU
>  	select ARCH_NO_PREEMPT
>  	select ARCH_NO_SG_CHAIN
>  	select ARCH_USE_CMPXCHG_LOCKREF
> diff --git a/arch/alpha/include/asm/percpu.h b/arch/alpha/include/asm/percpu.h
> index 6923249f2d49..5d8b75ec3bf2 100644
> --- a/arch/alpha/include/asm/percpu.h
> +++ b/arch/alpha/include/asm/percpu.h
> @@ -10,8 +10,8 @@
>   *
>   * Always use weak definitions for percpu variables in modules.
>   */
> -#if defined(MODULE) && defined(CONFIG_SMP)
> -#define ARCH_NEEDS_WEAK_PER_CPU
> +#if defined(MODULE) && defined(CONFIG_SMP) && defined(CONFIG_ARCH_NEEDS_WEAK_PER_CPU)
> +#define MODULE_NEEDS_WEAK_PER_CPU

CONFIG_ARCH_NEEDS_WEAK_PER_CPU is always set with the above select.
So there is no point in adding this to the check.

Furthermore this removes ARCH_NEEDS_WEAK_PER_CPU and defines
MODULE_NEEDS_WEAK_PER_CPU while the common code conversion happens
only with patch 4. Or in other words: if patches are split like this
things break.

Same is true for patch 3. Just merging patches 2-4 would be the
easiest solution to this problem.

Re: [PATCH 5/5] mm/alloc_tag: add the CONFIG_ARCH_NEEDS_WEAK_PER_CPU macro when statically defining the percpu variable _shared_alloc_tag

[Heiko Carstens]

On Thu, Jun 12, 2025 at 11:57:49AM +0300, Mike Rapoport wrote:
> On Thu, Jun 12, 2025 at 04:27:30PM +0800, Hao Ge wrote:
> > From: Hao Ge <gehao@kylinos.cn>
> > 
> > Recently discovered this entry while checking kallsyms on ARM64:
> > ffff800083e509c0 D _shared_alloc_tag
> > 
> > If CONFIG_ARCH_NEEDS_WEAK_PER_CPU is not defined(it is only defined for
> > s390 and alpha architectures),there's no need to statically define
> > the percpu variable _shared_alloc_tag. As the number of CPUs
> > increases,the wasted memory will grow correspondingly.
> > 
> > Enclose the definition of _shared_alloc_tag within the
> > CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.
> > 
> > Suggested-by: Suren Baghdasaryan <surenb@google.com>
> > Signed-off-by: Hao Ge <gehao@kylinos.cn>
> > ---
> >  lib/alloc_tag.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
> > index c7f602fa7b23..14fd66f26e42 100644
> > --- a/lib/alloc_tag.c
> > +++ b/lib/alloc_tag.c
> > @@ -24,8 +24,10 @@ static bool mem_profiling_support;
> >  
> >  static struct codetag_type *alloc_tag_cttype;
> >  
> > +#ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU
> 
> It should be enough to add #ifdef ARCH_NEEDS_WEAK_PER_CPU here instead of
> all the churn.

That won't work since ARCH_NEEDS_WEAK_PER_CPU is only defined if MODULE is
also defined, which is not the case for core kernel code like lib/alloc_tag.c.

As a side note: I'm wondering if s390 still needs ARCH_NEEDS_WEAK_PER_CPU due
to all the compiler option changes we had recently. But that's a different
story and independent of this series.

Re: [PATCH 2/5] alpha: Modify the definition logic of WEAK_PER_CPU

[Hao Ge]

On 2025/6/12 19:22, Heiko Carstens wrote:
> On Thu, Jun 12, 2025 at 04:27:27PM +0800, Hao Ge wrote:
>> From: Hao Ge <gehao@kylinos.cn>
>>
>> As stated in the first patch of this patch series,
>> we make ARCH_NEEDS_WEAK_PER_CPU a Kconfig option,
>> and replace all instances of ARCH_NEEDS_WEAK_PER_CPU
>> in the kernel code with MODULE_NEEDS_WEAK_PER_CPU,
>> gated by #ifdef CONFIG_ARCH_NEEDS_WEAK_PER_CPU.
>>
>> We'll make corresponding changes for the alpha
>> architecture.
>>
>> Suggested-by: Suren Baghdasaryan <surenb@google.com>
>> Signed-off-by: Hao Ge <gehao@kylinos.cn>
>> ---
>>   arch/alpha/Kconfig              | 1 +
>>   arch/alpha/include/asm/percpu.h | 4 ++--
>>   2 files changed, 3 insertions(+), 2 deletions(-)
> ...
>
>> +	select ARCH_NEEDS_WEAK_PER_CPU
>>   	select ARCH_NO_PREEMPT
>>   	select ARCH_NO_SG_CHAIN
>>   	select ARCH_USE_CMPXCHG_LOCKREF
>> diff --git a/arch/alpha/include/asm/percpu.h b/arch/alpha/include/asm/percpu.h
>> index 6923249f2d49..5d8b75ec3bf2 100644
>> --- a/arch/alpha/include/asm/percpu.h
>> +++ b/arch/alpha/include/asm/percpu.h
>> @@ -10,8 +10,8 @@
>>    *
>>    * Always use weak definitions for percpu variables in modules.
>>    */
>> -#if defined(MODULE) && defined(CONFIG_SMP)
>> -#define ARCH_NEEDS_WEAK_PER_CPU
>> +#if defined(MODULE) && defined(CONFIG_SMP) && defined(CONFIG_ARCH_NEEDS_WEAK_PER_CPU)
>> +#define MODULE_NEEDS_WEAK_PER_CPU

Hi Heiko

Thank you for taking the time to review these patches.

> CONFIG_ARCH_NEEDS_WEAK_PER_CPU is always set with the above select.
> So there is no point in adding this to the check.
Yes, that's exactly the case.
> Furthermore this removes ARCH_NEEDS_WEAK_PER_CPU and defines
> MODULE_NEEDS_WEAK_PER_CPU while the common code conversion happens
> only with patch 4. Or in other words: if patches are split like this
> things break.
>
> Same is true for patch 3. Just merging patches 2-4 would be the
> easiest solution to this problem.

I think this should be CC'd to the stable branch.

I'm wondering if these need to be integrated into a single patch.

I'm not sure. What do you think?


Hi Andrew and Suren

How do you see this? Any suggestions?

Thanks
Best Regards
Hao

Re: [PATCH 2/5] alpha: Modify the definition logic of WEAK_PER_CPU

[Heiko Carstens]

On Thu, Jun 12, 2025 at 08:06:25PM +0800, Hao Ge wrote:
> > Furthermore this removes ARCH_NEEDS_WEAK_PER_CPU and defines
> > MODULE_NEEDS_WEAK_PER_CPU while the common code conversion happens
> > only with patch 4. Or in other words: if patches are split like this
> > things break.
> > 
> > Same is true for patch 3. Just merging patches 2-4 would be the
> > easiest solution to this problem.
> 
> I think this should be CC'd to the stable branch.

Why should this go to stable? This is just a minor optimization.

> I'm wondering if these need to be integrated into a single patch.
> 
> I'm not sure. What do you think?

stable or not: this series must be bisectable, which is currently not the case.