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

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Message-ID: <5d2fec2b-8e59-417e-b9e6-12c6e27dd5f0@gaisler.com>
Date: Thu, 11 Sep 2025 07:38:15 +0200
From: Andreas Larsson <andreas@...sler.com>
To: Arnd Bergmann <arnd@...db.de>, ksummit@...ts.linux.dev
Cc: linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
 linuxppc-dev@...ts.ozlabs.org, linux-mips@...r.kernel.org,
 linux-mm@...ck.org, imx@...ts.linux.dev,
 Christophe Leroy <christophe.leroy@...roup.eu>,
 Richard Weinberger <richard@....at>, 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>, Vlastimil Babka
 <vbabka@...e.cz>, Suren Baghdasaryan <surenb@...gle.com>,
 Ira Weiny <ira.weiny@...el.com>, Nishanth Menon <nm@...com>,
 Heiko Stübner <heiko@...ech.de>,
 Alexander Sverdlin <alexander.sverdlin@...il.com>,
 "Chester A. Unal" <chester.a.unal@...nc9.com>,
 Sergio Paracuellos <sergio.paracuellos@...il.com>
Subject: Re: [TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

On 2025-09-09 23:23, Arnd Bergmann wrote:
> High memory is one of the least popular features of the Linux kernel.
> Added in 1999 for linux-2.3.16 to support large x86 machines, there
> are very few systems that still need it. I talked about about this
> recently at the Embedded Linux Conference on 32-bit systems [1][2][3]
> and there were a few older discussions before[4][5][6].
> 
> While removing a feature that is actively used is clearly a regression
> and not normally done, I expect removing highmem is going to happen
> at some point anyway when there are few enough users, but the question
> is when that time will be.
> 
> I'm still collecting information about which of the remaining highmem
> users plan to keep updating their kernels and for what reason. Some
> users obviously are alarmed about potentially losing this ability,
> so I hope to get a broad consensus on a specific timeline for how long
> we plan to support highmem in the page cache and to give every user
> sufficient time to migrate to a well-tested alternative setup if that
> is possible, or stay on a highmem-enabled LTS kernel for as long
> as necessary.

We have a upcoming SoC with support for up to 16 GiB of DRAM. When that is
used in LEON sparc32 configuration (using 36-bit physical addressing), a
removed CONFIG_HIGHMEM would be a considerable limitation, even after an
introduction of different CONFIG_VMSPLIT_* options for sparc32.

Regards,
Andreas