linux-kernel - Re: [PATCH v3] mm: Make SPLIT_PTE

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Message-ID: <05549846-d813-44ad-966e-e42001570d0e@redhat.com>
Date: Tue, 24 Sep 2024 22:10:50 +0200
From: David Hildenbrand <david@...hat.com>
To: Guenter Roeck <linux@...ck-us.net>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
 Geert Uytterhoeven <geert@...ux-m68k.org>
Subject: Re: [PATCH v3] mm: Make SPLIT_PTE_PTLOCKS depend on SMP

On 24.09.24 17:42, Guenter Roeck wrote:
> SPLIT_PTE_PTLOCKS depends on "NR_CPUS >= 4". Unfortunately, that evaluates
> to true if there is no NR_CPUS configuration option. This results in
> CONFIG_SPLIT_PTE_PTLOCKS=y for mac_defconfig. This in turn causes the m68k
> "q800" and "virt" machines to crash in qemu if debugging options are
> enabled.
> 
> Making CONFIG_SPLIT_PTE_PTLOCKS dependent on the existence of NR_CPUS
> does not work since a dependency on the existence of a numeric Kconfig
> entry always evaluates to false. Example:
> 
> config HAVE_NO_NR_CPUS
>         def_bool y
>         depends on !NR_CPUS
> 
> After adding this to a Kconfig file, "make defconfig" includes:
> $ grep NR_CPUS .config
> CONFIG_NR_CPUS=64
> CONFIG_HAVE_NO_NR_CPUS=y
> 
> Defining NR_CPUS for m68k does not help either since many architectures
> define NR_CPUS only for SMP configurations.
> 
> Make SPLIT_PTE_PTLOCKS depend on SMP instead to solve the problem.
> 
> Fixes: 394290cba966 ("mm: turn USE_SPLIT_PTE_PTLOCKS / USE_SPLIT_PTE_PTLOCKS into Kconfig options")
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
> Signed-off-by: Guenter Roeck <linux@...ck-us.net>
> ---

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb