linux-kernel - [PATCH 1/5] mm/Kconfig: add ARCH_NEEDS_WEAK_PER

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Message-Id: <c7e5c5d737c2373f18320e5589e1921c662f8e35.1749715979.git.gehao@kylinos.cn>
Date: Thu, 12 Jun 2025 16:27:26 +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 1/5] mm/Kconfig: add ARCH_NEEDS_WEAK_PER_CPU option

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. 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@...gle.com>
Signed-off-by: Hao Ge <gehao@...inos.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