mm/percpu: Conditionally define _shared_alloc_tag via CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU

PATCH Applied Hao Ge 2 participants 2 messages Jun 18, 2025 – Jun 19, 2025

Subsystems (7)

Memory management Library code MM core s390 +3 more subsystems

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Hao Ge

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2025-06-18 01:59 UTC #0 lore raw

PATCH v6 1/1 mm/percpu: Conditionally define _shared_alloc_tag via CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU

From: Hao Ge <[email protected]>

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.

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 remove all instances of ARCH_NEEDS_WEAK_PER_CPU from the code and introduced CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU to replace the relevant logic. We can now conditionally define the perpcu variable _shared_alloc_tag based on CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU. 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.

Acked-by Alexander Gordeev <[email protected]> # s390

Signed-off-by Hao Ge <[email protected]>

---
v6: Add Acked-by: Alexander Gordeev <[email protected]> # s390
As Suren pointed out, enclose defined(CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU) && defined(MODULE) part of the condition in parentheses.
As Andrew suggested, I'll consolidate it into a standalone patch. Thanks to Alexander, Suren, and Andrew.

v5: Regarding the omission of defined(MODULE) in alloc_tag.h where
only #ifdef CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU was used, I apologize for this error.
Please find version 5 attached to address this issue.

v4:
Merge previous patches into a single patch.
Remove all instances of ARCH_MODULE_NEEDS_WEAK_PER_CPU from v3 and use CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU to
replace the relevant logic.
Replace CONFIG_ARCH_NEEDS_WEAK_PER_CPU with CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU in v3, as weak percpu support is only required for modules ,making the name more semantically accurate.
David, Mike, Matthew, Kent, Heiko and Suren have all provided valuable input. Thanks for this.

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

---
 arch/alpha/Kconfig              | 1 +
 arch/alpha/include/asm/percpu.h | 5 ++---
 arch/s390/Kconfig               | 1 +
 arch/s390/include/asm/percpu.h  | 5 ++---
 include/linux/alloc_tag.h       | 6 +++---
 include/linux/percpu-defs.h     | 7 ++++---
 lib/alloc_tag.c                 | 2 ++
 mm/Kconfig                      | 7 +++++++
 8 files changed, 22 insertions(+), 12 deletions(-)

arch/alpha/Kconfig +1 -0

index 109a4cddcd13..80367f2cf821 100644
@@ -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_MODULE_NEEDS_WEAK_PER_CPU if SMP
 	select ARCH_NO_PREEMPT
 	select ARCH_NO_SG_CHAIN
 	select ARCH_USE_CMPXCHG_LOCKREF

arch/alpha/include/asm/percpu.h +2 -3

index 6923249f2d49..4383d66341dc 100644
@@ -9,10 +9,9 @@
  * way above 4G.
  *
  * Always use weak definitions for percpu variables in modules.
+ * Therefore, we have enabled CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU
+ * in the Kconfig.
  */
-#if defined(MODULE) && defined(CONFIG_SMP)
-#define ARCH_NEEDS_WEAK_PER_CPU
-#endif
 
 #include <asm-generic/percpu.h>

arch/s390/Kconfig +1 -0

index 0c16dc443e2f..b652cb952f31 100644
@@ -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_MODULE_NEEDS_WEAK_PER_CPU
 	select ARCH_STACKWALK
 	select ARCH_SUPPORTS_ATOMIC_RMW
 	select ARCH_SUPPORTS_DEBUG_PAGEALLOC

arch/s390/include/asm/percpu.h +2 -3

index 84f6b8357b45..96af7d964014 100644
@@ -16,10 +16,9 @@
  * For 64 bit module code, the module may be more than 4G above the
  * per cpu area, use weak definitions to force the compiler to
  * generate external references.
+ * Therefore, we have enabled CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU
+ * in the Kconfig.
  */
-#if defined(MODULE)
-#define ARCH_NEEDS_WEAK_PER_CPU
-#endif
 
 /*
  * We use a compare-and-swap loop since that uses less cpu cycles than

include/linux/alloc_tag.h +3 -3

index 8f7931eb7d16..9ef2633e2c08 100644
@@ -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
+#if defined(CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU) && defined(MODULE)
 /*
  * 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 /* CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU && MODULE */
 
 #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 /* CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU && MODULE */
 
 DECLARE_STATIC_KEY_MAYBE(CONFIG_MEM_ALLOC_PROFILING_ENABLED_BY_DEFAULT,
 			mem_alloc_profiling_key);

include/linux/percpu-defs.h +4 -3

index 0aeb0e276a3e..1669c9f4fd50 100644
@@ -63,14 +63,15 @@
  * 1. The symbol must be globally unique, even the static ones.
  * 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.
+ * Archs which need weak percpu definitions should set
+ * CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU 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(CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU) && defined(MODULE)) || \
+	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

lib/alloc_tag.c +2 -0

index c7f602fa7b23..ab0936ebf38e 100644
@@ -24,8 +24,10 @@ static bool mem_profiling_support;
 
 static struct codetag_type *alloc_tag_cttype;
 
+#ifdef CONFIG_ARCH_MODULE_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);

mm/Kconfig +7 -0

index e113f713b493..00514df3eae4 100644
@@ -929,6 +929,13 @@ config ARCH_SUPPORTS_PUD_PFNMAP
 	def_bool y
 	depends on ARCH_SUPPORTS_HUGE_PFNMAP && HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD
 
+#
+# Architectures that always use weak definitions for percpu
+# variables in modules should set this.
+#
+config ARCH_MODULE_NEEDS_WEAK_PER_CPU
+       bool
+
 #
 # UP and nommu archs use km based percpu allocator
 #

Signature · 1 line

2.25.1

Mike Rapoport

reviewer Acked-by on v6 1/1

2025-06-19 05:44 UTC #1 lore raw

Quoted 24 lines · Hao Ge

From: Hao Ge <[email protected]>
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.
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 remove all instances of ARCH_NEEDS_WEAK_PER_CPU from the code and introduced CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU to replace the relevant logic. We can now conditionally define the perpcu variable _shared_alloc_tag based on CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU. 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.

Quoted 3 lines

Suggested-by Suren Baghdasaryan <[email protected]>
Acked-by Alexander Gordeev <[email protected]> # s390
Signed-off-by Hao Ge <[email protected]>

Acked-by Mike Rapoport (Microsoft) <[email protected]>

Signature · 2 lines

Sincerely yours,
Mike.