linux-kernel - Re: [PATCH v10 01/25] mm: introduce CONFIG_SPECULATIVE_PAGE

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Date:   Mon, 23 Apr 2018 14:58:09 +0900
From:   Minchan Kim <minchan@...nel.org>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
Cc:     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>,
        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>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        kemi.wang@...el.com, sergey.senozhatsky.work@...il.com,
        Daniel Jordan <daniel.m.jordan@...cle.com>,
        David Rientjes <rientjes@...gle.com>,
        Jerome Glisse <jglisse@...hat.com>,
        Ganesh Mahendran <opensource.ganesh@...il.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        haren@...ux.vnet.ibm.com, khandual@...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 v10 01/25] mm: introduce CONFIG_SPECULATIVE_PAGE_FAULT

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@...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 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
>