Re: [PATCH 1/3] arch: Introduce ARCH_HAS_CPUMASK_BITS

[David Hildenbrand]

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@xxxxxxxxxx>
> ---
>   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