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

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Message-ID: <E986779F-C7AA-4940-9508-08601EE2FDD0@zytor.com>
Date: Fri, 12 Sep 2025 02:58:48 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andreas Larsson <andreas@...sler.com>, 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 September 10, 2025 10:38:15 PM PDT, Andreas Larsson <andreas@...sler.com> wrote:
>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
>
>

It really sounds like a self-inflicted problem... getting your customers switched over to the RV64 side is probably the best you can do for them.