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

[Vladimir Murzin]

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 at 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.

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

Cheers
Vladimir

> 
> Thanks,
> Dennis
>