Re: [PATCH v3] mm: Make SPLIT_PTE_PTLOCKS depend on SMP

[Geert Uytterhoeven]

On Tue, Sep 24, 2024 at 5:42 PM Guenter Roeck <linux@xxxxxxxxxxxx> 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@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
> Signed-off-by: Guenter Roeck <linux@xxxxxxxxxxxx>

Reviewed-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Tested-by: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>

> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -595,6 +595,7 @@ config ARCH_MHP_MEMMAP_ON_MEMORY_ENABLE
>  config SPLIT_PTE_PTLOCKS
>         def_bool y
>         depends on MMU
> +       depends on SMP
>         depends on NR_CPUS >= 4

Nit: please combine these two lines, to make it clear they are related.

>         depends on !ARM || CPU_CACHE_VIPT
>         depends on !PARISC || PA20

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds