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

[Dennis Zhou <dennis@kernel.org>]

On Tue, Dec 14, 2021 at 08:02:58PM +0100, Geert Uytterhoeven wrote:
> Hi Dennis,
> 
> On Tue, Dec 14, 2021 at 6:26 PM Dennis Zhou <dennis@kernel.org> wrote:
> > On Tue, Dec 14, 2021 at 05:29:22PM +0100, Geert Uytterhoeven wrote:
> > > 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?
> >
> > It's not great.. Can you share the line on boot with the following
> > prefix: pcpu-alloc [1].
> 
> There are no such lines.
> "make mm/percpu.i mm/percpu.s" and inspecting the generated files,
> and vmlinux, proves the code is there. But apparently it's not called.
> 
> So there may be no issue on my system?
> 

I might be missing something, but that can't be right. Percpu calls
pcpu_dump_alloc_info() from pcpu_setup_first_chunk() which is called by
both embed/page first chunk code.

Ummm. That can't be right. Percpu call pcpu_dump_alloc_info() from
pcpu_setup_first_chunk() which everyone should call. On my machine:

$ dmesg | grep "pcpu-alloc"
[    0.065118] pcpu-alloc: s184320 r8192 d28672 u262144 alloc=1*2097152

Thanks,
Dennis