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

[Andrew Morton]

On Tue, 27 Jan 2026 04:07:04 +0900 Ryusuke Konishi <konishi.ryusuke@xxxxxxxxx> wrote:

> 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")

Thanks.  b505f1944535 had cc:stable so let's add some cc's to alert
-stable maintainers.

I see that b505f1944535 prevented a Xen warning, but did it have any
other runtime effects?  If not, a prompt revert may be the way to
proceed for now.

> 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