linux-kernel - Re: [RFC PATCH v5 13/18] mm: Map page tables with privileged pkey

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Message-ID: <e4f546cd-67c0-4198-975d-266a7cb9807a@arm.com>
Date: Tue, 19 Aug 2025 11:35:01 +0200
From: Kevin Brodsky <kevin.brodsky@....com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>,
 "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
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Subject: Re: [RFC PATCH v5 13/18] mm: Map page tables with privileged pkey

On 18/08/2025 19:01, Edgecombe, Rick P wrote:
> On Mon, 2025-08-18 at 18:02 +0200, Kevin Brodsky wrote:
>> The benchmarking results (see cover letter) don't seem to point to a
>> major performance hit from setting the pkey on arm64 (worth noting that
>> the linear mapping is PTE-mapped on arm64 today so no splitting should
>> occur when setting the pkey). The overhead may well be substantially
>> higher on x86.
> It's surprising to me. The batching seems to be about switching the pkey, not
> the conversion of the direct map.

Correct, there is still a set_memory_pkey() for each PTP.

>  And with batching you measured a fork
> benchmark actually sped up a tiny bit. Shouldn't it involve a pile of page table
> allocations and so extra direct map work?

It should indeed...

> I don't know if it's possible the mock implementation skipped some set_memory()
> work somehow?

In fact you're absolutely right, in the mock implementation I
benchmarked set_memory_pkey() is in fact a no-op :( This is because
patch 6 gates set_memory_pkey() on system_supports_poe(), but the mock
implementation [1] only modifies arch_kpkeys_enabled(). In other words
the numbers in the cover letter correspond to the added pkey register
switches, without touching the page tables.

I am now re-running the benchmarks with set_memory_pkey() actually
modifying the page tables. I'll reply to the cover letter with the
updated numbers.

- Kevin

[1]
https://gitlab.arm.com/linux-arm/linux-kb/-/commit/fd75b43abb354e84d06f3dfb05ce839e9fb13e08