Re: [PATCH v12 01/31] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT
From: Jerome Glisse
Date: Thu Apr 18 2019 - 17:47:32 EST
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@xxxxxxxxxxxxx>
> Suggested-by: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: Laurent Dufour <ldufour@xxxxxxxxxxxxx>
Reviewed-by: Jérôme Glisse <jglisse@xxxxxxxxxx>
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
> +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.