linux-kernel - Re: [TECH TOPIC] Reaching consensus on CONFIG

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Message-ID: <dec53524-97ee-4e56-8795-c7549c295fac@sr71.net>
Date: Wed, 10 Sep 2025 09:34:43 -0700
From: Dave Hansen <dave@...1.net>
To: Richard Weinberger <richard@....at>, Arnd Bergmann <arnd@...db.de>
Cc: ksummit <ksummit@...ts.linux.dev>,
 linux-kernel <linux-kernel@...r.kernel.org>,
 linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
 linuxppc-dev@...ts.ozlabs.org, linux-mips <linux-mips@...r.kernel.org>,
 linux-mm <linux-mm@...ck.org>, imx@...ts.linux.dev,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Lucas Stach <l.stach@...gutronix.de>,
 Linus Walleij <linus.walleij@...aro.org>,
 Geert Uytterhoeven <geert+renesas@...der.be>,
 Ankur Arora <ankur.a.arora@...cle.com>, David Hildenbrand
 <david@...hat.com>, Mike Rapoport <rppt@...nel.org>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Matthew Wilcox <willy@...radead.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>, vbabka <vbabka@...e.cz>,
 Suren Baghdasaryan <surenb@...gle.com>, Ira Weiny <ira.weiny@...el.com>,
 Nishanth Menon <nm@...com>, heiko <heiko@...ech.de>,
 Alexander Sverdlin <alexander.sverdlin@...il.com>,
 "Chester A. Unal" <chester.a.unal@...nc9.com>,
 Sergio Paracuellos <sergio.paracuellos@...il.com>,
 Andreas Larsson <andreas@...sler.com>
Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

On 9/10/25 07:04, Richard Weinberger wrote:
> I am part of a team responsible for products based on various 32-bit
> SoCs, so I'm alarmed. These products, which include ARMv7 and PPC32
> architectures with up to 2 GiB of RAM, are communication systems
> with five-figure deployments worldwide.
> 
> Removing high memory will have an impact on these systems. The
> oldest kernel version they run is 4.19 LTS, with upgrades to a more
> recent LTS release currently in progress. We typically upgrade the
> kernel every few years and will continue to support these systems
> for at least the next 10 years.
> 
> Even with a new memory split, which could utilize most of the
> available memory, I expect there to be issues with various
> applications and FPGA device drivers.
I'd be really curious what the _actual_ issues would be with a
non-standard split. There are a lot of "maybe" problems and solutions
here, but it's hard to move forward without known practical problems to
tackle.

Has anybody run into actual end user visible problems when using one of
weirdo PAGE_OFFSET configs?