Some architectures set and maintain the mm_cpumask bits when loading
or removing process from cpu.
This Kconfig will mark those to allow different behavior between
kernels that maintain the mm_cpumask and those that do not.

Signed-off-by: Yair Podemsky <ypodemsk@redhat.com>
---
 arch/Kconfig         | 8 ++++++++
 arch/arm/Kconfig     | 1 +
 arch/powerpc/Kconfig | 1 +
 arch/s390/Kconfig    | 1 +
 arch/sparc/Kconfig   | 1 +
 arch/x86/Kconfig     | 1 +
 6 files changed, 13 insertions(+)

diff --git a/arch/Kconfig b/arch/Kconfig
index e3511afbb7f2..ec5559779e9f 100644
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -1434,6 +1434,14 @@ config ARCH_HAS_NONLEAF_PMD_YOUNG
 	  address translations. Page table walkers that clear the accessed bit
 	  may use this capability to reduce their search space.
 
+config ARCH_HAS_CPUMASK_BITS
+	bool
+	help
+	  Architectures that select this option set bits on the mm_cpumask
+	  to mark which cpus loaded the mm, The mask can then be used to
+	  control mm specific actions such as tlb_flush.
+
+
 source "kernel/gcov/Kconfig"
 
 source "scripts/gcc-plugins/Kconfig"
diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index e24a9820e12f..6111059a68a3 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -70,6 +70,7 @@ config ARM
 	select GENERIC_SCHED_CLOCK
 	select GENERIC_SMP_IDLE_THREAD
 	select HARDIRQS_SW_RESEND
+	select ARCH_HAS_CPUMASK_BITS
 	select HAVE_ARCH_AUDITSYSCALL if AEABI && !OABI_COMPAT
 	select HAVE_ARCH_BITREVERSE if (CPU_32v7M || CPU_32v7) && !CPU_32v6
 	select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL && !CPU_ENDIAN_BE32 && MMU
diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
index a6c4407d3ec8..2fd0160f4f8e 100644
--- a/arch/powerpc/Kconfig
+++ b/arch/powerpc/Kconfig
@@ -144,6 +144,7 @@ config PPC
 	select ARCH_HAS_TICK_BROADCAST		if GENERIC_CLOCKEVENTS_BROADCAST
 	select ARCH_HAS_UACCESS_FLUSHCACHE
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
+	select ARCH_HAS_CPUMASK_BITS
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select ARCH_KEEP_MEMBLOCK
 	select ARCH_MIGHT_HAVE_PC_PARPORT
diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
index 9809c74e1240..b2de5ee07faf 100644
--- a/arch/s390/Kconfig
+++ b/arch/s390/Kconfig
@@ -86,6 +86,7 @@ config S390
 	select ARCH_HAS_SYSCALL_WRAPPER
 	select ARCH_HAS_UBSAN_SANITIZE_ALL
 	select ARCH_HAS_VDSO_DATA
+	select ARCH_HAS_CPUMASK_BITS
 	select ARCH_HAVE_NMI_SAFE_CMPXCHG
 	select ARCH_INLINE_READ_LOCK
 	select ARCH_INLINE_READ_LOCK_BH
diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
index 84437a4c6545..f9e0cf26d447 100644
--- a/arch/sparc/Kconfig
+++ b/arch/sparc/Kconfig
@@ -98,6 +98,7 @@ config SPARC64
 	select ARCH_HAS_PTE_SPECIAL
 	select PCI_DOMAINS if PCI
 	select ARCH_HAS_GIGANTIC_PAGE
+	select ARCH_HAS_CPUMASK_BITS
 	select HAVE_SOFTIRQ_ON_OWN_STACK
 	select HAVE_SETUP_PER_CPU_AREA
 	select NEED_PER_CPU_EMBED_FIRST_CHUNK
diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index a825bf031f49..d98dfdf9c6b4 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -183,6 +183,7 @@ config X86
 	select HAVE_ARCH_THREAD_STRUCT_WHITELIST
 	select HAVE_ARCH_STACKLEAK
 	select HAVE_ARCH_TRACEHOOK
+	select ARCH_HAS_CPUMASK_BITS
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE
 	select HAVE_ARCH_TRANSPARENT_HUGEPAGE_PUD if X86_64
 	select HAVE_ARCH_USERFAULTFD_WP         if X86_64 && USERFAULTFD
-- 
2.31.1

On 04.04.23 15:42, Yair Podemsky wrote:
> Some architectures set and maintain the mm_cpumask bits when loading
> or removing process from cpu.
> This Kconfig will mark those to allow different behavior between
> kernels that maintain the mm_cpumask and those that do not.
> 

I was wondering if we should do something along the lines of:

diff --git a/include/linux/mm_types.h b/include/linux/mm_types.h
index 0722859c3647..1f5c15d8e8ed 100644
--- a/include/linux/mm_types.h
+++ b/include/linux/mm_types.h
@@ -767,11 +767,13 @@ struct mm_struct {
  #endif /* CONFIG_LRU_GEN */
         } __randomize_layout;

+#ifdef CONFIG_MM_CPUMASK
         /*
          * The mm_cpumask needs to be at the end of mm_struct, because it
          * is dynamically sized based on nr_cpu_ids.
          */
         unsigned long cpu_bitmap[];
+#endif
  };

But that would, of course, require additional changes to make it 
compile. What concerns me a bit is that we have in mm/rmap.c a 
mm_cpumask() usage. But it's glued to 
CONFIG_ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH ... shaky.

At least if we would properly fence it, there would be no
accidental abuse anymore.


> Signed-off-by: Yair Podemsky <ypodemsk@redhat.com>
> ---
>   arch/Kconfig         | 8 ++++++++
>   arch/arm/Kconfig     | 1 +
>   arch/powerpc/Kconfig | 1 +
>   arch/s390/Kconfig    | 1 +
>   arch/sparc/Kconfig   | 1 +
>   arch/x86/Kconfig     | 1 +

As Valentin says, there are other architectures that do the same.

-- 
Thanks,

David / dhildenb