linux-kernel - Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE

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Message-ID: <alpine.DEB.2.20.1803280310380.68839@chino.kir.corp.google.com>
Date:   Wed, 28 Mar 2018 03:16:55 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     Laurent Dufour <ldufour@...ux.vnet.ibm.com>
cc:     Thomas Gleixner <tglx@...utronix.de>, paulmck@...ux.vnet.ibm.com,
        peterz@...radead.org, akpm@...ux-foundation.org,
        kirill@...temov.name, ak@...ux.intel.com, mhocko@...nel.org,
        dave@...olabs.net, jack@...e.cz,
        Matthew Wilcox <willy@...radead.org>, benh@...nel.crashing.org,
        mpe@...erman.id.au, paulus@...ba.org,
        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>,
        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,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        linuxppc-dev@...ts.ozlabs.org, x86@...nel.org
Subject: Re: [PATCH v9 01/24] mm: Introduce CONFIG_SPECULATIVE_PAGE_FAULT

On Wed, 28 Mar 2018, 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.
> >>
> >> Suggested-by: Thomas Gleixner <tglx@...utronix.de>
> >> Signed-off-by: Laurent Dufour <ldufour@...ux.vnet.ibm.com>
> >> ---
> >>  mm/Kconfig | 3 +++
> >>  1 file changed, 3 insertions(+)
> >>
> >> diff --git a/mm/Kconfig b/mm/Kconfig
> >> index abefa573bcd8..07c566c88faf 100644
> >> --- a/mm/Kconfig
> >> +++ b/mm/Kconfig
> >> @@ -759,3 +759,6 @@ config GUP_BENCHMARK
> >>  	  performance of get_user_pages_fast().
> >>  
> >>  	  See tools/testing/selftests/vm/gup_benchmark.c
> >> +
> >> +config SPECULATIVE_PAGE_FAULT
> >> +       bool
> > 
> > Should this be configurable even if the arch supports it?
> 
> Actually, this is not configurable unless by manually editing the .config file.
> 
> I made it this way on the Thomas's request :
> https://lkml.org/lkml/2018/1/15/969
> 
> That sounds to be the smarter way to achieve that, isn't it ?
> 

Putting this in mm/Kconfig is definitely the right way to go about it 
instead of any generic option in arch/*.

My question, though, was making this configurable by the user:

config SPECULATIVE_PAGE_FAULT
	bool "Speculative page faults"
	depends on X86_64 || PPC
	default y
	help
	  ..

It's a question about whether we want this always enabled on x86_64 and 
power or whether the user should be able to disable it (right now they 
can't).  With a large feature like this, you may want to offer something 
simple (disable CONFIG_SPECULATIVE_PAGE_FAULT) if someone runs into 
regressions.