linux-kernel - Re: [PATCH 3/3] x86/mm: Move _PAGE_BIT

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Message-ID: <yv2sjut5urpzz426z4wmufshbron35ohwtqk6jzun46elbcqtf@3z54nh33yl4m>
Date: Thu, 23 Oct 2025 15:24:17 +0100
From: Kiryl Shutsemau <kas@...nel.org>
To: Dave Hansen <dave.hansen@...el.com>
Cc: Usama Arif <usamaarif642@...il.com>, dwmw@...zon.co.uk, 
	tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com, 
	ardb@...nel.org, hpa@...or.com, x86@...nel.org, apopple@...dia.com, 
	thuth@...hat.com, nik.borisov@...e.com, linux-kernel@...r.kernel.org, 
	linux-efi@...r.kernel.org, kernel-team@...a.com, 
	Michael van der Westhuizen <rmikey@...a.com>, Tobias Fleig <tfleig@...a.com>
Subject: Re: [PATCH 3/3] x86/mm: Move _PAGE_BIT_NOPTISHADOW from bit 58 to
 bit 9

On Thu, Oct 23, 2025 at 07:05:24AM -0700, Dave Hansen wrote:
> On 10/22/25 16:58, Usama Arif wrote:
> >> This isn't necessary once the previous 2 patches are applied, right?
> > In kexec if the target kernels have patch 1 and 2, then this patch
> > is not needed. Unfortunately, patches 1 and 2 are not livepatchable.
> > Also backporting patches 1 and 2 to all previous kernels running in
> > production in a large fleet is not very scalable.
> 
> I don't think I've ever been asked to apply a patch to make livepatching
> easier. I'm not sure that's something we want to pollute mainline with.

It is not about assisting livepatching.

Machines in our fleet may switch between kernel versions using kexec.

We recently introduced a kernel in the fleet that enables 5-level
paging.

Kexecing into an older kernel that requires switching from 5- to 4-level
paging which is broken because the target kernel doesn't expect
_PAGE_NOPTISHADOW.

The first two patches fix the problem for the target kernel. If we only
apply them upstream, we would need to backport them to all kernels we
use to address the problem.

The last patch allows us to only update the kernel that has 5-level
paging enabled, making it much easier logistically.

The fix seems trivial, and I don't see any downsides.

Ultimately, it helps with interoperability between different kernel
versions and/or configurations.

-- 
  Kiryl Shutsemau / Kirill A. Shutemov