Re: [PATCH v10 01/25] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT

[Minchan Kim <minchan@kernel.org>]

Hi Laurent,

I guess it's good timing to review. Guess LSF/MM goes so might change
a lot since then. :) Anyway, I grap a time to review.

On Tue, Apr 17, 2018 at 04:33:07PM +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, SMP and MMU.

Can we have description in here why it depends on architecture?

> 
> 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 d5004d82a1d6..5484dca11199 100644
> --- a/mm/Kconfig
> +++ b/mm/Kconfig
> @@ -752,3 +752,25 @@ config GUP_BENCHMARK
>  	  performance of get_user_pages_fast().
>  
>  	  See tools/testing/selftests/vm/gup_benchmark.c
> +
> +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 MMU && SMP
> +       help
> +         Try to handle user space page faults without holding the mmap_sem.
> +
> +	 This should allow better concurrency for massively threaded process
> +	 since the page fault handler will not wait for other threads 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 of 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
> +	 is then tried.
> -- 
> 2.7.4
>