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

From: Rob Landley <rob@landley.net>
To: Dennis Zhou <dennis@kernel.org>,
	Vladimir Murzin <vladimir.murzin@arm.com>
Cc: linux-arch-owner@vger.kernel.org, linux-mm@kvack.org,
	tj@kernel.org, cl@linux.com, akpm@linux-foundation.org,
	npiggin@gmail.com, hch@lst.de, arnd@arndb.de,
	linux-sh@vger.kernel.org, dalias@libc.org,
	linux-riscv@lists.infradead.org
Subject: Re: [PATCH] percpu: km: ensure it is used with NOMMU (either UP or SMP)
Date: Mon, 6 Dec 2021 06:01:59 -0600	[thread overview]
Message-ID: <a2473a90-bdd3-b5af-6967-73c59952b7c1@landley.net> (raw)
In-Reply-To: <YaqFyznA5hab3PUA@fedora>

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>

Rob