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Subject: Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT
To:     David Rientjes <rientjes@google.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, paulmck@linux.vnet.ibm.com,
        peterz@infradead.org, akpm@linux-foundation.org,
        kirill@shutemov.name, ak@linux.intel.com, mhocko@kernel.org,
        dave@stgolabs.net, jack@suse.cz,
        Matthew Wilcox <willy@infradead.org>, benh@kernel.crashing.org,
        mpe@ellerman.id.au, paulus@samba.org,
        Ingo Molnar <mingo@redhat.com>, hpa@zytor.com,
        Will Deacon <will.deacon@arm.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Alexei Starovoitov <alexei.starovoitov@gmail.com>,
        kemi.wang@intel.com, sergey.senozhatsky.work@gmail.com,
        Daniel Jordan <daniel.m.jordan@oracle.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        haren@linux.vnet.ibm.com, khandual@linux.vnet.ibm.com,
        npiggin@gmail.com, bsingharora@gmail.com,
        Tim Chen <tim.c.chen@linux.intel.com>,
        linuxppc-dev@lists.ozlabs.org, x86@kernel.org
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From:   Laurent Dufour <ldufour@linux.vnet.ibm.com>
Date:   Wed, 28 Mar 2018 13:15:25 +0200
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On 28/03/2018 12:16, David Rientjes wrote:
> On Wed, 28 Mar 2018, Laurent Dufour wrote:
> 
>>>> This configuration variable will be used to build the code needed to
>>>> handle speculative page fault.
>>>>
>>>> By default it is turned off, and activated depending on architecture
>>>> support.
>>>>
>>>> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
>>>> Signed-off-by: Laurent Dufour <ldufour@linux.vnet.ibm.com>
>>>> ---
>>>>  mm/Kconfig | 3 +++
>>>>  1 file changed, 3 insertions(+)
>>>>
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index abefa573bcd8..07c566c88faf 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -759,3 +759,6 @@ config GUP_BENCHMARK
>>>>  	  performance of get_user_pages_fast().
>>>>  
>>>>  	  See tools/testing/selftests/vm/gup_benchmark.c
>>>> +
>>>> +config SPECULATIVE_PAGE_FAULT
>>>> +       bool
>>>
>>> Should this be configurable even if the arch supports it?
>>
>> Actually, this is not configurable unless by manually editing the .config file.
>>
>> I made it this way on the Thomas's request :
>> https://lkml.org/lkml/2018/1/15/969
>>
>> That sounds to be the smarter way to achieve that, isn't it ?
>>
> 
> Putting this in mm/Kconfig is definitely the right way to go about it 
> instead of any generic option in arch/*.
> 
> My question, though, was making this configurable by the user:
> 
> config SPECULATIVE_PAGE_FAULT
> 	bool "Speculative page faults"
> 	depends on X86_64 || PPC
> 	default y
> 	help
> 	  ..
> 
> It's a question about whether we want this always enabled on x86_64 and 
> power or whether the user should be able to disable it (right now they 
> can't).  With a large feature like this, you may want to offer something 
> simple (disable CONFIG_SPECULATIVE_PAGE_FAULT) if someone runs into 
> regressions.

I agree, but I think it would be important to get the per architecture
enablement to avoid complex check here. For instance in the case of powerPC
this is only supported for PPC_BOOK3S_64.

To avoid exposing such per architecture define here, what do you think about
having supporting architectures setting ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
and the SPECULATIVE_PAGE_FAULT depends on this, like this:

In mm/Kconfig:
config SPECULATIVE_PAGE_FAULT
 	bool "Speculative page faults"
 	depends on ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT && SMP
 	default y
 	help
		...

In arch/powerpc/Kconfig:
config PPC
	...
	select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT	if PPC_BOOK3S_64

In arch/x86/Kconfig:
config X86_64
	...
	select ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT