Subject: Re: [PATCH v12 01/31] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT

From: Jerome Glisse <[email-masked]>
Date: Thu, 18 Apr 2019 17:47:21 -0400

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 <[email-masked]>
> Suggested-by: David Rientjes <[email-masked]>
> Signed-off-by: Laurent Dufour <[email-masked]>

Reviewed-by: Jérôme Glisse <[email-masked]>

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 :)

> +	  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
>