Re: [PATCH] mm: Make SPLIT_PTE_PTLOCKS depend on the existence of NR_CPUS

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Günter,

CC kbuild

I have two comments...

On Tue, Sep 24, 2024 at 1:52 AM Guenter Roeck <linux@roeck-us.net> wrote:
> On 9/23/24 15:08, Guenter Roeck wrote:
> > On 9/23/24 08:23, David Hildenbrand wrote:
> >> On 23.09.24 16:25, Guenter Roeck wrote:
> >>> SPLIT_PTE_PTLOCKS already depends on "NR_CPUS >= 4", but that evaluates
> >>> to true if there is no NR_CPUS configuration option (such as for m68k).
> >>> This results in CONFIG_SPLIT_PTE_PTLOCKS=y for mac_defconfig.
> >>> This in turn causes the m68k "q800" machine to crash in qemu.

Should this be fixed in Kconfig (too)?

> >> Oh, that's why my compile tests still worked ... I even removed the additional NR_CPUS check, assuming it's not required ...
> >>
> >> Thanks for debugging and fixing!
> >>
> >> Acked-by: David Hildenbrand <david@redhat.com>
> >>
> >
> > Apparently it wasn't that simple :-(. 0-day reports a build failure
> > with s390 builds.
> >
> > arch/s390/mm/gmap.c:357:16: error: implicit declaration of function 'pmd_pgtable_page'.
> >
> > Turns out that
> >      depends on NR_CPUS && NR_CPUS >= 4
> >
> > doesn't work and disables SPLIT_PTE_PTLOCKS even if NR_CPUS _is_ defined.
> > I have no idea how to declare the dependency correctly.
> > Sorry, I did not expect that.
>
> The only solution I found was to define NR_CPUS for m68k. That seems to be
> the only architecture not defining it, so hopefully that is an acceptable
> solution. I'll send v2 of the patch shortly.

My first thought was to agree, as m68k is indeed the only architecture
that does not define NR_CPUS. Upon closer look, most architectures
have NR_CPUS depend on SMP, hence I assume the issue could happen for
those too (although I didn't manage to create such a config on anything
but m68k)?  So the simple solution would be to add a dependency on
SMP to SPLIT_PTE_PTLOCKS.

BTW, the list of excluded architectures looks fragile to me:

    config SPLIT_PTE_PTLOCKS
            def_bool y
            depends on MMU
            depends on NR_CPUS >= 4
            depends on !ARM || CPU_CACHE_VIPT
            depends on !PARISC || PA20
            depends on !SPARC32

If this can't be handled in a generic way, perhaps this should be
changed from opt-out to opt-in (i.e. select gate symbol in arch-specific
Kconfig)?

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

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