linux-kernel - Re: [PATCH 05/11] x86: remove HIGHMEM64G support

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Message-Id: <2615bf9e-61a4-488e-8842-3230de4f5033@app.fastmail.com>
Date: Sat, 12 Apr 2025 12:40:13 +0200
From: "Arnd Bergmann" <arnd@...db.de>
To: "Dave Hansen" <dave.hansen@...el.com>, "Arnd Bergmann" <arnd@...nel.org>,
 linux-kernel@...r.kernel.org, x86@...nel.org
Cc: "Thomas Gleixner" <tglx@...utronix.de>, "Ingo Molnar" <mingo@...hat.com>,
 "Borislav Petkov" <bp@...en8.de>,
 "Dave Hansen" <dave.hansen@...ux.intel.com>,
 "H. Peter Anvin" <hpa@...or.com>,
 "Linus Torvalds" <torvalds@...ux-foundation.org>,
 "Andy Shevchenko" <andy@...nel.org>, "Matthew Wilcox" <willy@...radead.org>,
 "Sean Christopherson" <seanjc@...gle.com>,
 "Davide Ciminaghi" <ciminaghi@...dd.com>,
 "Paolo Bonzini" <pbonzini@...hat.com>, kvm@...r.kernel.org,
 "Mike Rapoport" <rppt@...nel.org>, "Mike Rapoport" <rppt@...nel.org>
Subject: Re: [PATCH 05/11] x86: remove HIGHMEM64G support

On Sat, Apr 12, 2025, at 01:44, Dave Hansen wrote:
> Has anyone run into any problems on 6.15-rc1 with this stuff?
>
> 0xf75fe000 is the mem_map[] entry for the first page >4GB. It obviously
> wasn't allocated, thus the oops. Looks like the memblock for the >4GB
> memory didn't get removed although the pgdats seem correct.
>
> I'll dig into it some more. Just wanted to make sure there wasn't a fix
> out there already.
>
> The way I'm triggering this is booting qemu with a 32-bit PAE kernel,
> and "-m 4096" (or more).

I have reproduced the bug now and found that it did not happen in
my series. Bisection points to Mike Rapoport's highmem series,
specifically  6faea3422e3b ("arch, mm: streamline HIGHMEM freeing")

There was a related bug that was caused by an earlier version
of my series when I also removed CONFIG_PHYS_ADDR_T_64BIT
https://lore.kernel.org/all/202412201005.77fb063-lkp@intel.com/

    Arnd