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

[Dennis Zhou <dennis@kernel.org>]

On Mon, Dec 06, 2021 at 11:21:46AM -0500, Rich Felker wrote:
> On Mon, Dec 06, 2021 at 06:01:59AM -0600, Rob Landley wrote:
> > On 12/3/21 3:02 PM, Dennis Zhou wrote:
> > > 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.
> > 
> > I can't test against current until I get some unrelated fixes from Rich Felker
> > (who's been busy over the weekend), but I tested the "depends" version on 5.10
> > and got a shell prompt on my "make ARCH=sh j2_defconfig" board.
> > 
> > Tested-by: Rob Landley <rob@landley.net>
> 
> Tested-by: Rich Felker <dalias@libc.org>
> 
> I've tested the version as originally posted and it works for j2 (sh2)
> (both build and runtime checked). I'd tested the other (depends
> version) before and I'm fine with either going upstream.
> 
> Rich
> 

Thank you all for testing and bearing with me. I've applied this to
for-5.16-fixes. Will send it up later in the week.

Thanks,
Dennis