Re: [PATCH] percpu: km: ensure it is used with NOMMU (either UP or SMP)

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

Hi Vladimir and Dennis,

On Wed, Dec 1, 2021 at 12:53 PM Vladimir Murzin <vladimir.murzin@arm.com> wrote:
> On 11/30/21 5:41 PM, Dennis Zhou wrote:
> > On Tue, Nov 30, 2021 at 05:29:54PM +0000, Vladimir Murzin wrote:
> >> Currently, NOMMU pull km allocator via !SMP dependency because most of
> >> them are UP, yet for SMP+NOMMU vm allocator gets pulled which:
> >>
> >> * may lead to broken build [1]
> >> * ...or not working runtime due to [2]
> >>
> >> It looks like SMP+NOMMU case was overlooked in bbddff054587 ("percpu:
> >> use percpu allocator on UP too") so restore that.
> >>
> >> [1]
> >> For ARM SMP+NOMMU (R-class cores)
> >>
> >> arm-none-linux-gnueabihf-ld: mm/percpu.o: in function `pcpu_post_unmap_tlb_flush':
> >> mm/percpu-vm.c:188: undefined reference to `flush_tlb_kernel_range'
> >>
> >> [2]
> >> static inline
> >> int vmap_pages_range_noflush(unsigned long addr, unsigned long end,
> >>                 pgprot_t prot, struct page **pages, unsigned int page_shift)
> >> {
> >>        return -EINVAL;
> >> }
> >>
> >> Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
> >> ---
> >>  mm/Kconfig | 3 +--
> >>  1 file changed, 1 insertion(+), 2 deletions(-)
> >>
> >> diff --git a/mm/Kconfig b/mm/Kconfig
> >> index d16ba92..66331e0 100644
> >> --- a/mm/Kconfig
> >> +++ b/mm/Kconfig
> >> @@ -425,9 +425,8 @@ config THP_SWAP
> >>  # UP and nommu archs use km based percpu allocator
> >>  #
> >>  config NEED_PER_CPU_KM
> >> -    depends on !SMP
> >>      bool
> >> -    default y
> >> +    default !SMP || !MMU
> >>
> >
> > Should this be `depends on !SMP || !MMU` with default yes? Because with
> > SMP && MMU, it shouldn't be an option to run with percpu-km.
>
> IIUC these are equivalent, truth table would not change if is under "depends"
> or "default"
>
> SMP    MMU   NEED_PER_CPU_KM
>  y      y    !y || !y => n || n => n
>  y      n    !y || !n => n || y => y
>  n      y    !n || !y => y || n => y
>  n      n    !n || !n => y || y => y
>
> >
> >>  config CLEANCACHE
> >>      bool "Enable cleancache driver to cache clean pages if tmem is present"
> >> --
> >> 2.7.4
> >>
> >
> > It's interesting to me that this is all coming up at once. Earlier this
> > month I had the same conversation with people involved with sh [1].
> >
> > [1] https://lore.kernel.org/linux-sh/YY7tp5attRyK42Zk@fedora/
> >
> > I can pull this shortly once I see whatever happened to linux-sh.
>
> Ahh, good to know! Adding SH folks here (start of discussion [0]). I see you came
> to the same conclusion, right?
>
> IIRC, RISC-V also have SMP+NOMMU, so adding them as well.

I had seen the j-Core thread, but completely forgot about
Canaan K210 (RV64 SMP+NOMMU).

This became commit 3583521aabac76e5 ("percpu: km: ensure it is used
with NOMMU (either UP or SMP)").  And now booting K210 prints:

    percpu: wasting 10 pages per chunk

a) Is this bad?
b) What exactly was this fixing, and how would I trigger the bad case
   on K210 before, if it was affected at all?

>
> [0] https://lore.kernel.org/linux-mm/20211130172954.129587-1-vladimir.murzin@arm.com/T/

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