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

[Dennis Zhou <dennis@kernel.org>]

On Wed, Dec 01, 2021 at 11:51:04AM +0000, Vladimir Murzin wrote:
> Hi,
> 
> On 11/30/21 5:41 PM, Dennis Zhou wrote:
> > Hello,
> > 
> > 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
> 

I may be wrong, but I think this is slightly different as we're using
#ifdef / #if defined().

> > 
> >>  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? 
> 

Yeah, I don't see anything else from linux-sh. So I'll go ahead and
apply this with my change if you're fine with that.

> IIRC, RISC-V also have SMP+NOMMU, so adding them as well.
> 
> [0] https://lore.kernel.org/linux-mm/20211130172954.129587-1-vladimir.murzin@arm.com/T/
> 
> Cheers
> Vladimir
> 

Thanks,
Dennis