Re: [PATCH v10 24/25] x86/mm: add speculative pagefault handling

From: Punit Agrawal <punit.agrawal@arm.com>
To: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Cc: Punit Agrawal <punitagrawal@gmail.com>,
	akpm@linux-foundation.org, mhocko@kernel.org,
	peterz@infradead.org, kirill@shutemov.name, ak@linux.intel.com,
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	benh@kernel.crashing.org, mpe@ellerman.id.au, paulus@samba.org,
	Thomas Gleixner <tglx@linutronix.de>,
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	Tim Chen <tim.c.ch en@linux.intel.com>,
	linuxppc-dev@lists.ozlabs.org, x86@kernel.org
Subject: Re: [PATCH v10 24/25] x86/mm: add speculative pagefault handling
Date: Fri, 04 May 2018 16:55:25 +0100	[thread overview]
Message-ID: <87efirl8k2.fsf@e105922-lin.cambridge.arm.com> (raw)
In-Reply-To: <fb143123-d54e-b08d-1bd8-07767c86c7d0@linux.vnet.ibm.com> (Laurent Dufour's message of "Thu, 3 May 2018 16:59:14 +0200")

Laurent Dufour <ldufour@linux.vnet.ibm.com> writes:

> On 30/04/2018 20:43, Punit Agrawal wrote:
>> Hi Laurent,
>> 
>> I am looking to add support for speculative page fault handling to
>> arm64 (effectively porting this patch) and had a few questions.
>> Apologies if I've missed an obvious explanation for my queries. I'm
>> jumping in bit late to the discussion.
>
> Hi Punit,
>
> Thanks for giving this series a review.
> I don't have arm64 hardware to play with, but I'll be happy to add arm64
> patches to my series and to try to maintain them.

I'll be happy to try them on arm64 platforms I have access to and
provide feedback.

>
>> 
>> On Tue, Apr 17, 2018 at 3:33 PM, Laurent Dufour
>> <ldufour@linux.vnet.ibm.com> wrote:
>>> From: Peter Zijlstra <peterz@infradead.org>
>>>

[...]

>>>
>>> -       vma = find_vma(mm, address);
>>> +       if (!vma || !can_reuse_spf_vma(vma, address))
>>> +               vma = find_vma(mm, address);
>> 
>> Is there a measurable benefit from reusing the vma?
>> 
>> Dropping the vma reference unconditionally after speculative page
>> fault handling gets rid of the implicit state when "vma != NULL"
>> (increased ref-count). I found it a bit confusing to follow.
>
> I do agree, this is quite confusing. My initial goal was to be able to reuse
> the VMA in the case a protection key error was detected, but it's not really
> necessary on x86 since we know at the beginning of the fault operation that
> protection key are in the loop. This is not the case on ppc64 but I couldn't
> find a way to easily rely on the speculatively fetched VMA neither, so for
> protection keys, this didn't help.
>
> Regarding the measurable benefit of reusing the fetched vma, I did further
> tests using will-it-scale/page_fault2_threads test, and I'm no more really
> convince that this worth the added complexity. I think I'll drop the patch "mm:
> speculative page fault handler return VMA" of the series, and thus remove the
> call to can_reuse_spf_vma().

Makes sense. Thanks for giving this a go.

Punit

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