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Subject: Re: [PATCH v12 01/31] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT
To:     Jerome Glisse <jglisse@redhat.com>
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References: <20190416134522.17540-1-ldufour@linux.ibm.com>
 <20190416134522.17540-2-ldufour@linux.ibm.com>
 <20190418214721.GA11645@redhat.com>
From:   Laurent Dufour <ldufour@linux.ibm.com>
Date:   Tue, 23 Apr 2019 17:21:37 +0200
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Le 18/04/2019 à 23:47, Jerome Glisse a écrit :
> On Tue, Apr 16, 2019 at 03:44:52PM +0200, 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, ARCH_HAS_PTE_SPECIAL, SMP and MMU.
>>
>> The architecture support is needed since the speculative page fault handler
>> is called from the architecture's page faulting code, and some code has to
>> be added there to handle the speculative handler.
>>
>> The dependency on ARCH_HAS_PTE_SPECIAL is required because vm_normal_page()
>> does processing that is not compatible with the speculative handling in the
>> case ARCH_HAS_PTE_SPECIAL is not set.
>>
>> Suggested-by: Thomas Gleixner <tglx@linutronix.de>
>> Suggested-by: David Rientjes <rientjes@google.com>
>> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> 
> Reviewed-by: Jérôme Glisse <jglisse@redhat.com>

Thanks Jérôme.

> Small question below
> 
>> ---
>>   mm/Kconfig | 22 ++++++++++++++++++++++
>>   1 file changed, 22 insertions(+)
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index 0eada3f818fa..ff278ac9978a 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -761,4 +761,26 @@ config GUP_BENCHMARK
>>   config ARCH_HAS_PTE_SPECIAL
>>   	bool
>>   
>> +config ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
>> +       def_bool n
>> +
>> +config SPECULATIVE_PAGE_FAULT
>> +	bool "Speculative page faults"
>> +	default y
>> +	depends on ARCH_SUPPORTS_SPECULATIVE_PAGE_FAULT
>> +	depends on ARCH_HAS_PTE_SPECIAL && MMU && SMP
>> +	help
>> +	  Try to handle user space page faults without holding the mmap_sem.
>> +
>> +	  This should allow better concurrency for massively threaded processes
> 
> Is there any case where it does not provide better concurrency ? The
> should make me wonder :)

Depending on the VMA's type, it may not provide better concurrency. 
Indeed only anonymous mapping are managed currently. Perhaps this should 
be mentioned here, is it ?

>> +	  since the page fault handler will not wait for other thread's memory
>> +	  layout change to be done, assuming that this change is done in
>> +	  another part of the process's memory space. This type of page fault
>> +	  is named speculative page fault.
>> +
>> +	  If the speculative page fault fails because a concurrent modification
>> +	  is detected or because underlying PMD or PTE tables are not yet
>> +	  allocated, the speculative page fault fails and a classic page fault
>> +	  is then tried.
>> +
>>   endmenu
>> -- 
>> 2.21.0
>>
>