mm: Restrict the static definition of the per-CPU variable _shared_alloc_tag to s390 and alpha architectures only

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. 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/[email protected]/
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