linux-kernel - [REGRESSION] x86_32 boot hang in 6.19-rc7 caused by b505f1944535 ("x86/kfence: avoid writing L1TF-vulnerable PTEs")

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Message-ID: <CAKFNMokwjw68ubYQM9WkzOuH51wLznHpEOMSqtMoV1Rn9JV_gw@mail.gmail.com>
Date: Tue, 27 Jan 2026 04:07:04 +0900
From: Ryusuke Konishi <konishi.ryusuke@...il.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Marco Elver <elver@...gle.com>, 
	LKML <linux-kernel@...r.kernel.org>, Alexander Potapenko <glider@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, X86 ML <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Jann Horn <jannh@...gle.com>, kasan-dev@...glegroups.com
Subject: [REGRESSION] x86_32 boot hang in 6.19-rc7 caused by b505f1944535
 ("x86/kfence: avoid writing L1TF-vulnerable PTEs")

Hi All,

I am reporting a boot regression in v6.19-rc7 on an x86_32
environment. The kernel hangs immediately after "Booting the kernel"
and does not produce any early console output.

A git bisect identified the following commit as the first bad commit:
b505f1944535 ("x86/kfence: avoid writing L1TF-vulnerable PTEs")

Environment and Config:
- Guest Arch: x86_32  (one of my test VMs)
- Memory Config: # CONFIG_X86_PAE is not set
- KFENCE Config: CONFIG_KFENCE=y
- Host/Hypervisor: x86_64 host running KVM

The system fails to boot at a very early stage. I have confirmed that
reverting commit b505f1944535 on top of v6.19-rc7 completely resolves
the issue, and the kernel boots normally.

Could you please verify if this change is compatible with x86_32
(non-PAE) configurations?
I am happy to provide my full .config or test any potential fixes.

Best regards,
Ryusuke Konishi