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Message-Id: <20250618015809.1235761-1-hao.ge@linux.dev>
Date: Wed, 18 Jun 2025 09:58:09 +0800
From: Hao Ge <hao.ge@...ux.dev>
To: Andrew Morton <akpm@...ux-foundation.org>,
David Hildenbrand <david@...hat.com>,
Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
"Liam R. Howlett" <Liam.Howlett@...cle.com>,
Vlastimil Babka <vbabka@...e.cz>,
Suren Baghdasaryan <surenb@...gle.com>,
Mike Rapoport <rppt@...nel.org>,
Richard Henderson <richard.henderson@...aro.org>,
Matt Turner <mattst88@...il.com>,
Dennis Zhou <dennis@...nel.org>,
Tejun Heo <tj@...nel.org>,
Christoph Lameter <cl@...ux.com>,
Heiko Carstens <hca@...ux.ibm.com>,
Vasily Gorbik <gor@...ux.ibm.com>,
Alexander Gordeev <agordeev@...ux.ibm.com>,
Christian Borntraeger <borntraeger@...ux.ibm.com>,
Sven Schnelle <svens@...ux.ibm.com>,
Kent Overstreet <kent.overstreet@...ux.dev>
Cc: linux-mm@...ck.org,
linux-kernel@...r.kernel.org,
linux-alpha@...r.kernel.org,
linux-s390@...r.kernel.org,
Hao Ge <hao.ge@...ux.dev>,
Hao Ge <gehao@...inos.cn>
Subject: [PATCH v6] mm/percpu: Conditionally define _shared_alloc_tag via CONFIG_ARCH_MODULE_NEEDS_WEAK_PER_CPU
From: Hao Ge <gehao@...inos.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.
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.
Suggested-by: Suren Baghdasaryan <surenb@...gle.com>
Acked-by: Alexander Gordeev <agordeev@...ux.ibm.com> # s390
Signed-off-by: Hao Ge <gehao@...inos.cn>
---
v6: Add Acked-by: Alexander Gordeev <agordeev@...ux.ibm.com> # 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(-)
diff --git a/arch/alpha/Kconfig b/arch/alpha/Kconfig
index 109a4cddcd13..80367f2cf821 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_MODULE_NEEDS_WEAK_PER_CPU if SMP
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..4383d66341dc 100644
--- a/arch/alpha/include/asm/percpu.h
+++ b/arch/alpha/include/asm/percpu.h
@@ -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>
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 0c16dc443e2f..b652cb952f31 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_MODULE_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..96af7d964014 100644
--- a/arch/s390/include/asm/percpu.h
+++ b/arch/s390/include/asm/percpu.h
@@ -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
diff --git a/include/linux/alloc_tag.h b/include/linux/alloc_tag.h
index 8f7931eb7d16..9ef2633e2c08 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
+#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);
diff --git a/include/linux/percpu-defs.h b/include/linux/percpu-defs.h
index 0aeb0e276a3e..1669c9f4fd50 100644
--- a/include/linux/percpu-defs.h
+++ b/include/linux/percpu-defs.h
@@ -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
diff --git a/lib/alloc_tag.c b/lib/alloc_tag.c
index c7f602fa7b23..ab0936ebf38e 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_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);
diff --git a/mm/Kconfig b/mm/Kconfig
index e113f713b493..00514df3eae4 100644
--- a/mm/Kconfig
+++ b/mm/Kconfig
@@ -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
#
--
2.25.1
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