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Date:   Tue, 4 Apr 2023 15:47:56 +0200
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Subject: Re: [PATCH 1/3] arch: Introduce ARCH_HAS_CPUMASK_BITS
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To:     Yair Podemsky <ypodemsk@redhat.com>, linux@armlinux.org.uk,
        mpe@ellerman.id.au, npiggin@gmail.com, christophe.leroy@csgroup.eu,
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Cc:     alougovs@redhat.com
References: <20230404134224.137038-1-ypodemsk@redhat.com>
 <20230404134224.137038-2-ypodemsk@redhat.com>
From:   David Hildenbrand <david@redhat.com>
Organization: Red Hat
In-Reply-To: <20230404134224.137038-2-ypodemsk@redhat.com>
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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