linux-kernel - Re: [PATCH v11 01/26] mm: introduce CONFIG_SPECULATIVE_PAGE

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Date:   Tue, 22 May 2018 13:47:08 +0200
From:   Laurent Dufour <ldufour@...ux.vnet.ibm.com>
To:     Randy Dunlap <rdunlap@...radead.org>, akpm@...ux-foundation.org,
        mhocko@...nel.org, peterz@...radead.org, kirill@...temov.name,
        ak@...ux.intel.com, dave@...olabs.net, jack@...e.cz,
        Matthew Wilcox <willy@...radead.org>,
        khandual@...ux.vnet.ibm.com, aneesh.kumar@...ux.vnet.ibm.com,
        benh@...nel.crashing.org, mpe@...erman.id.au, paulus@...ba.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, hpa@...or.com,
        Will Deacon <will.deacon@....com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        sergey.senozhatsky.work@...il.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        Minchan Kim <minchan@...nel.org>,
        Punit Agrawal <punitagrawal@...il.com>,
        vinayak menon <vinayakm.list@...il.com>,
        Yang Shi <yang.shi@...ux.alibaba.com>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, npiggin@...il.com, bsingharora@...il.com,
        paulmck@...ux.vnet.ibm.com, Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v11 01/26] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT

On 17/05/2018 18:36, Randy Dunlap wrote:
> Hi,
> 
> On 05/17/2018 04:06 AM, 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@...utronix.de>
>> Suggested-by: David Rientjes <rientjes@...gle.com>
>> Signed-off-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
>> ---
>>  mm/Kconfig | 22 ++++++++++++++++++++++
>>  1 file changed, 22 insertions(+)
>>
>> diff --git a/mm/Kconfig b/mm/Kconfig
>> index 1d0888c5b97a..a38796276113 100644
>> --- a/mm/Kconfig
>> +++ b/mm/Kconfig
>> @@ -761,3 +761,25 @@ 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 process
> 
> 	                                                             processes
> 
>> +	 since the page fault handler will not wait for other threads memory
> 
> 	                                                      thread's
> 
>> +	 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 of a concurrency is
> 
> 	                                     because a concurrency is
> 
>> +	 detected or because underlying PMD or PTE tables are not yet
>> +	 allocating, it is failing its processing and a classic page fault
> 
> 	 allocated, the speculative page fault fails and a classic page fault
> 
>> +	 is then tried.
> 
> 
> Also, all of the help text (below the "help" line) should be indented by
> 1 tab + 2 spaces (in coding-style.rst).

Thanks, Randy for reviewing my miserable English grammar.
I'll fix that and the indentation.