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

Laurent Dufour [off-list ref] writes:

On 30/04/2018 20:43, Punit Agrawal wrote:

quoted

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.

quoted

On Tue, Apr 17, 2018 at 3:33 PM, Laurent Dufour
[off-list ref] wrote:

quoted

From: Peter Zijlstra <peterz@infradead.org>

[...]

quoted

quoted

-       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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