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

[Vladimir Murzin <vladimir.murzin@arm.com>]

On 12/3/21 9:02 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.

depends on !SMP || !MMU, also works for me, so feel free to amend. 

Thanks
Vladimir
> 
>> 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
>