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

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Message-ID: <797c78f4-1a90-42da-9fed-e87682456a43@gmail.com>
Date: Thu, 23 Oct 2025 00:58:15 +0100
From: Usama Arif <usamaarif642@...il.com>
To: Dave Hansen <dave.hansen@...el.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
Cc: x86@...nel.org, apopple@...dia.com, thuth@...hat.com,
 nik.borisov@...e.com, kas@...nel.org, 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 23/10/2025 00:35, Dave Hansen wrote:
> On 10/22/25 15:06, Usama Arif wrote:
>> Instead, move _PAGE_BIT_NOPTISHADOW to use _PAGE_BIT_SOFTW1 (bit 9),
> 
> Wait a sec, though...
> 
> 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.

So if we want to run a kernel with 5 level pagetable in production
(with the ability to kexec into a 4 level kernel that doesn't have the first
2 patches), then this patch would solve the problem. i.e. patches 1 and 2
solve the problem from the target kernels perspective, patch 3 solves
it from the source kernel (if the target kernel doesnt have patches 1
and 2 applied).
I mentioned this in the commit message as:

"
- The logistical challenge of patching all older kernels in production
- The patch not being applicable for live patching
"

I can try and make the commit message clearer in the next revision.