[PATCH v3 0/2] mm: Restrict the static definition of the per-CPU variable _shared_alloc_tag to s390 and alpha architectures only

[PATCH v3 0/2] 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,MODULE_NEEDS_WEAK_PER_CPU might
be a more accurate description,because it was only needed for modules.
Then,when defining the percpu variable _shared_alloc_tag,wrap it with the
CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.

The following version can be regarded as the most original version:
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

v3:
    Suren pointed out that patches 1-2 can be merged into a single patch
    in version 2. And the commit message for patch 3 can be made more
    concise.Make corresponding modifications based on the pointed-out
    issues and update the corresponding commit message.

v2:
    Heiko pointed out that when defining MODULE_NEEDS_WEAK_PER_CPU,
    the CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition in the v1 version
    should be removed,as it is always true for s390 and alpha
    architectures.And He also pointed out that patches 2-4 need to
    be merged into one patch. Modify the code according to the suggestions
    and update the corresponding commit message.

Hao Ge (2):
  mm: Optimize the ARCH_NEEDS_WEAK_PER_CPU logic for s390/alpha
    architectures
  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 | 2 +-
 arch/s390/Kconfig               | 1 +
 arch/s390/include/asm/percpu.h  | 2 +-
 include/linux/alloc_tag.h       | 6 +++---
 include/linux/percpu-defs.h     | 4 ++--
 lib/alloc_tag.c                 | 2 ++
 mm/Kconfig                      | 4 ++++
 8 files changed, 15 insertions(+), 7 deletions(-)

-- 
2.25.1

[PATCH v3 1/2] mm: Optimize the ARCH_NEEDS_WEAK_PER_CPU logic for s390/alpha architectures

[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.

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,MODULE_NEEDS_WEAK_PER_CPU might
be a more accurate description,because it was only needed for modules.
Then,when defining the percpu variable _shared_alloc_tag,wrap it
with the CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.

Therefore, this patch does the following things:
Add the ARCH_NEEDS_WEAK_PER_CPU option to the mm Kconfig file
and enable it for the s390 and alpha architectures.
And replace all instances of ARCH_NEEDS_WEAK_PER_CPU
in the kernel code with MODULE_NEEDS_WEAK_PER_CPU.

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 | 2 +-
 arch/s390/Kconfig               | 1 +
 arch/s390/include/asm/percpu.h  | 2 +-
 include/linux/alloc_tag.h       | 6 +++---
 include/linux/percpu-defs.h     | 4 ++--
 mm/Kconfig                      | 4 ++++
 7 files changed, 13 insertions(+), 7 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..b164d3720e9e 100644
--- a/arch/alpha/include/asm/percpu.h
+++ b/arch/alpha/include/asm/percpu.h
@@ -11,7 +11,7 @@
  * Always use weak definitions for percpu variables in modules.
  */
 #if defined(MODULE) && defined(CONFIG_SMP)
-#define ARCH_NEEDS_WEAK_PER_CPU
+#define MODULE_NEEDS_WEAK_PER_CPU
 #endif
 
 #include <asm-generic/percpu.h>
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..0afc7ce9c26f 100644
--- a/arch/s390/include/asm/percpu.h
+++ b/arch/s390/include/asm/percpu.h
@@ -18,7 +18,7 @@
  * generate external references.
  */
 #if defined(MODULE)
-#define ARCH_NEEDS_WEAK_PER_CPU
+#define MODULE_NEEDS_WEAK_PER_CPU
 #endif
 
 /*
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
diff --git a/mm/Kconfig b/mm/Kconfig
index e113f713b493..2f55cc95cfcb 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 should 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 v3 2/2] 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>

Given the introduction of the CONFIG_ARCH_NEEDS_WEAK_PER_CPU macro,
we can now conditionally define the perpcu variable _shared_alloc_tag
based on its value. This allows architectures(such as s390/alpha) that
require weak definitions for percpu variables in modules to include
the definition,while others can omit it via compile-time exclusion.

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 v3 1/2] mm: Optimize the ARCH_NEEDS_WEAK_PER_CPU logic for s390/alpha architectures

[Matthew Wilcox]

On Mon, Jun 16, 2025 at 10:29:17AM +0800, Hao Ge wrote:
> +++ 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 should be enabled,see comments for DECLARE_PER_CPU_SECTION

This comment is inappropriate and should be removed.

> +config ARCH_NEEDS_WEAK_PER_CPU
> +       bool
> +
>  #
>  # UP and nommu archs use km based percpu allocator
>  #
> -- 
> 2.25.1
> 
> 

Re: [PATCH v3 1/2] mm: Optimize the ARCH_NEEDS_WEAK_PER_CPU logic for s390/alpha architectures

[Kent Overstreet]

On Mon, Jun 16, 2025 at 04:13:55AM +0100, Matthew Wilcox wrote:
> On Mon, Jun 16, 2025 at 10:29:17AM +0800, Hao Ge wrote:
> > +++ 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 should be enabled,see comments for DECLARE_PER_CPU_SECTION
> 
> This comment is inappropriate and should be removed.

Inappropriate? That's not a word I'm accustomed to hearing as an
engineering justification.

It's referring the reader to context they might want to know about, and
other comments they might want to read. Looks like a good comment to me,
if perhaps a bit terse.

Re: [PATCH v3 1/2] mm: Optimize the ARCH_NEEDS_WEAK_PER_CPU logic for s390/alpha architectures

[David Hildenbrand]

On 16.06.25 04:29, Hao Ge wrote:
> From: Hao Ge <gehao@kylinos.cn>

subject is misleading: we are not optimizing anything in this patch, do we?

It should probably be called

"mm/percpu: rename ARCH_NEEDS_WEAK_PER_CPU to MODULE_NEED_WEAK_PER_CPU" 
or sth. like that.

> 
> 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

"... 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.
 > > Therefore,we need to implement isolation for this purpose.

In general, throw in a space after "." and "," to make this easier to parse.

> 
> 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,MODULE_NEEDS_WEAK_PER_CPU might
> be a more accurate description,because it was only needed for modules.
> Then,when defining the percpu variable _shared_alloc_tag,wrap it
> with the CONFIG_ARCH_NEEDS_WEAK_PER_CPU condition.
> 
> Therefore, this patch does the following things:

"this patch" should be avoided.

"Therefore, let's ..."

> Add the ARCH_NEEDS_WEAK_PER_CPU option to the mm Kconfig file
> and enable it for the s390 and alpha architectures.
> And replace all instances of ARCH_NEEDS_WEAK_PER_CPU
> in the kernel code with MODULE_NEEDS_WEAK_PER_CPU.

Most of the description here should likely go to patch #2. See below.

> 
> 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 | 2 +-
>   arch/s390/Kconfig               | 1 +
>   arch/s390/include/asm/percpu.h  | 2 +-
>   include/linux/alloc_tag.h       | 6 +++---
>   include/linux/percpu-defs.h     | 4 ++--
>   mm/Kconfig                      | 4 ++++
>   7 files changed, 13 insertions(+), 7 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..b164d3720e9e 100644
> --- a/arch/alpha/include/asm/percpu.h
> +++ b/arch/alpha/include/asm/percpu.h
> @@ -11,7 +11,7 @@
>    * Always use weak definitions for percpu variables in modules.
>    */
>   #if defined(MODULE) && defined(CONFIG_SMP)
> -#define ARCH_NEEDS_WEAK_PER_CPU
> +#define MODULE_NEEDS_WEAK_PER_CPU
>   #endif
>   
>   #include <asm-generic/percpu.h>
> 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..0afc7ce9c26f 100644
> --- a/arch/s390/include/asm/percpu.h
> +++ b/arch/s390/include/asm/percpu.h
> @@ -18,7 +18,7 @@
>    * generate external references.
>    */
>   #if defined(MODULE)
> -#define ARCH_NEEDS_WEAK_PER_CPU
> +#define MODULE_NEEDS_WEAK_PER_CPU
>   #endif
>   
>   /*
> 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
> diff --git a/mm/Kconfig b/mm/Kconfig
> index e113f713b493..2f55cc95cfcb 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 should be enabled,see comments for DECLARE_PER_CPU_SECTION
> +config ARCH_NEEDS_WEAK_PER_CPU
> +       bool

I agree with Willy that this commit is not ideal -- in particular the 
"s390 and alpha should be enabled" can get stale easily.

If you want to add a comment, rather give guidance why any architecture 
would want to set this, like:

/* Architectures that XXX should set this. */

XXX to be defined.

But the bigger problem with this patch is that XXX cannot even be 
properly defined, because this patch does not make use of 
CONFIG_ARCH_NEEDS_WEAK_PER_CPU at all.

... that is done in the second patch.


So what you could do is move the actual introduction of 
CONFIG_ARCH_NEEDS_WEAK_PER_CPU to patch #2, where it is actually used, 
and limit this patch to the rename.

Similarly, teak the patch description to reflect only that.

-- 
Cheers,

David / dhildenb

Re: [PATCH v3 1/2] mm: Optimize the ARCH_NEEDS_WEAK_PER_CPU logic for s390/alpha architectures

[Mike Rapoport]

On Mon, Jun 16, 2025 at 09:59:09AM +0200, David Hildenbrand wrote:
> On 16.06.25 04:29, Hao Ge wrote:
> > From: Hao Ge <gehao@kylinos.cn>
> 
> subject is misleading: we are not optimizing anything in this patch, do we?
> 
> It should probably be called
> 
> "mm/percpu: rename ARCH_NEEDS_WEAK_PER_CPU to MODULE_NEED_WEAK_PER_CPU" or
> sth. like that.
> 
> 
> > Add the ARCH_NEEDS_WEAK_PER_CPU option to the mm Kconfig file
> > and enable it for the s390 and alpha architectures.
> > And replace all instances of ARCH_NEEDS_WEAK_PER_CPU
> > in the kernel code with MODULE_NEEDS_WEAK_PER_CPU.
> 
> Most of the description here should likely go to patch #2. See below.

...

> So what you could do is move the actual introduction of
> CONFIG_ARCH_NEEDS_WEAK_PER_CPU to patch #2, where it is actually used, and
> limit this patch to the rename.
> 
> Similarly, teak the patch description to reflect only that.

Right, if the patch only renames ARCH_NEEDS_WEAK_PER_CPU to
MODULE_NEEDS_WEAK_PER_CPU the description can be as simple as 

mm/percpu: rename ARCH_NEEDS_WEAK_PER_CPU to MODULE_NEEDS_WEAK_PER_CPU

as a preparation for introduction of CONFIG_ARCH_NEEDS_WEAK_PER_CPU.
No functional changes.
 
> -- 
> Cheers,
> 
> David / dhildenb
> 

-- 
Sincerely yours,
Mike.

Re: [PATCH v3 1/2] mm: Optimize the ARCH_NEEDS_WEAK_PER_CPU logic for s390/alpha architectures

[Suren Baghdasaryan]

On Mon, Jun 16, 2025 at 1:55 AM Mike Rapoport <rppt@kernel.org> wrote:
>
> On Mon, Jun 16, 2025 at 09:59:09AM +0200, David Hildenbrand wrote:
> > On 16.06.25 04:29, Hao Ge wrote:
> > > From: Hao Ge <gehao@kylinos.cn>
> >
> > subject is misleading: we are not optimizing anything in this patch, do we?
> >
> > It should probably be called
> >
> > "mm/percpu: rename ARCH_NEEDS_WEAK_PER_CPU to MODULE_NEED_WEAK_PER_CPU" or
> > sth. like that.
> >
> >
> > > Add the ARCH_NEEDS_WEAK_PER_CPU option to the mm Kconfig file
> > > and enable it for the s390 and alpha architectures.
> > > And replace all instances of ARCH_NEEDS_WEAK_PER_CPU
> > > in the kernel code with MODULE_NEEDS_WEAK_PER_CPU.
> >
> > Most of the description here should likely go to patch #2. See below.
>
> ...
>
> > So what you could do is move the actual introduction of
> > CONFIG_ARCH_NEEDS_WEAK_PER_CPU to patch #2, where it is actually used, and
> > limit this patch to the rename.
> >
> > Similarly, teak the patch description to reflect only that.
>
> Right, if the patch only renames ARCH_NEEDS_WEAK_PER_CPU to
> MODULE_NEEDS_WEAK_PER_CPU the description can be as simple as
>
> mm/percpu: rename ARCH_NEEDS_WEAK_PER_CPU to MODULE_NEEDS_WEAK_PER_CPU
>
> as a preparation for introduction of CONFIG_ARCH_NEEDS_WEAK_PER_CPU.
> No functional changes.

Yeah, the title is misleading and the description is too complicated.

Mike's suggested title sounds better to me and for description I would
say something like:

ARCH_NEEDS_WEAK_PER_CPU is currently defined only for modules and
therefore fails to represent requirements of the architecture. This
prevents us using it for conditions which are applicable when building
both modules and the kernel. To handle such conditions, make it a
Kconfig option and add MODULE_NEEDS_WEAK_PER_CPU for the cases when
the condition applies only to modules.

And now that I'm looking at the change I realize that we probably
don't even need a separate MODULE_NEEDS_WEAK_PER_CPU. It will be used
only in one place and can be replaced with:

#if defined(CONFIG_ARCH_NEEDS_WEAK_PER_CPU) && defined(MODULE)

The code inside arch/{alpha|s390}/include/asm/percpu.h that defines
MODULE_NEEDS_WEAK_PER_CPU can be completely removed and in
arch/alpha/Kconfig you can have:

select ARCH_NEEDS_WEAK_PER_CPU if CONFIG_SMP

to preserve CONFIG_SMP dependency.
That seems to me like a nicer cleanup.

>
> > --
> > Cheers,
> >
> > David / dhildenb
> >
>
> --
> Sincerely yours,
> Mike.