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Subject: Re: [PATCH v11 01/26] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT
To:     Laurent Dufour <ldufour@linux.vnet.ibm.com>,
        akpm@linux-foundation.org, mhocko@kernel.org, peterz@infradead.org,
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From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <2cb8256d-5822-d94d-b0e6-c46f21d84852@infradead.org>
Date:   Thu, 17 May 2018 09:36:00 -0700
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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@linutronix.de>
> Suggested-by: David Rientjes <rientjes@google.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.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).


-- 
~Randy