[TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

[H. Peter Anvin]

On September 9, 2025 2:23:37 PM PDT, Arnd Bergmann <arnd at arndb.de> 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.
>
>These are the assumptions I'm making, based on both what I have
>presented in my talk and feedback I have received so far, let me
>know if something is missing here:
>
>- Highmem in new kernels is almost exclusively an embedded Linux
>  topic. While there were a few late 32-bit desktop and laptop
>  systems that had more than 2GB of RAM, these were fairly short
>  lived and have long been unsupported by both the originally
>  shipping operating systems and most Linux distros. Notable
>  Examples include Pentium 4 "Northwood" desktops (sold 2003-2004),
>  Core Duo laptops (2006-2007), and Arm Chromebooks (rk3288,
>  Tegra k1, Exynos 5800, sold 2014-2017). Some PowerPC G4 Macs
>  and Atom Netbooks could be upgraded to 2GB.
>  There are a small number of users, but they really love these
>  devices and want to keep them alive, especially since they
>  mark the peak of the respective 32-bit product lines.
>
>- Within the embedded market, highmem is mostly used on ARMv7
>  based SoCs, but a few others also need it and still get kernel
>  updates:
>  PowerPC 85xx, 86xx and QoriQ P1/P2/P3/P4 (produced 2003-2021)
>  were used in some long-lived embedded systems with 2GB of RAM
>  or more. Mediatek MT7621 (MIPS32r3, introduced 2014 but still
>  sold) needs highmem to reach the upper 64MB of the 512MB physical
>  memory. Ingenic JZ4780 (MIPS32, released 2012) was used in the
>  short-lived MIPS Creator CI20 with 1GB of RAM (256MB lowmem)
>  and probably othes. Sparc32/LEON seems to be limited to 192MB of
>  lowmem as a kernel design choice. Vortex86DX3 supports 2GB
>  DDR3 memory.
>  The kernel also supports highmem on ARMv4, ARMv5, ARMv6,
>  PPC4xx, PPC82xx, PPC83xx, ARC, CSKY, Microblaze and Xtensa,
>  as well as additional MIPS SoCs,  but so far I could not find
>  any indication of any such machine with more than 1GB that
>  keeps getting kernel updates.
>
>- The vast majority of new embedded ARMv7 machines have 1GB of
>  RAM or less, which on many SoCs is a physical limitation
>  a narrow DDR3 memory interface, as well as a cost tradeoff.
>  The 1GB case is interesting because that usually means having
>  only 768MB of lowmem plus 256MB of highmem, as well as 3GB
>  of virtual addressing. I expect that almost all applications
>  on these work just as well with CONFIG_VMSPLIT_3G_OPT, changing
>  the limit to 1GB of lowmem and 2816MB of user address space.
>  The same thing should work on x86 and powerpc (CONFIG_LOWMEM_SIZE)
>  but not on mips and sparc where the limit is not configurable.
>
>- A few Arm SoCs have sparse physical address ranges for
>  their RAM, e.g. range per memory controllers like the Renesas
>  R-Car Gen 2 or Broadcom BCM4708. These currently require highmem
>  even on configurations with less than 1GB RAM, until we change
>  the way that sparsemem is handled to support rearranging the
>  linear map to fill all of lowmem. This still needs more work.
>
>- ARMv7 machines with 2GB remain in production, in particular
>  with the popular i.MX6Dual/Quad chip that has a 64-bit wide DDR3
>  interface and guaranteed manufacturer support until 2035.
>  This is the configuration I expect to see struggle the most.
>  Setting CONFIG_VMSPLIT_2G or CONFIG_VMSPLIT_2G_OPT should work
>  for most of the users but can break if an application runs out
>  of virtual address space, so this does require extensive testing,
>  and possibly user space changes. An example of possibly affected
>  userspace is Firefox, which needs more address space than other
>  applications but can perhaps be replaced with another embedded
>  browser.
>  
>- ARMv7 machines with 4GB and more exist and keep getting
>  kernel upgrades, but to my knowledge are not in production any
>  more. These are mainly 2010-2015 era chips based on rare
>  out-of-order cores like A15, A17 or PJ4 that were designed for
>  low-end servers, chromebooks and network equipment but replaced
>  with 64-bit chips shortly after. We had planned to bring a
>  CONFIG_VMSPLIT_4G_4G option to ARMv7VE to keep supporting the full
>  memory at a performance penalty, but currently have no plan to
>  finish this (volunteers welcome).
>  There is still some hope to keep them working with a combination
>  of CONFIG_VMSPLIT_2G and a modified ZRAM that can use high
>  pages without CONFIG_HIGHMEM, but whether this works depends
>  a lot on the application. I expect most of these products to
>  stop getting kernel updates in the next few years due to aging
>  hardware and increasing cost for updating out-of-tree patches
>  on platforms that are not fully upstream. I do not remember any
>  such devices with official support beyond 2030.
>
>My proposal based on the above assumptions is to gradually phase
>out highmem over the next 2 years for mainline kernels, obviously
>both the individual items and the timeline are completely up for
>debate:
>
>1. mark CONFIG_HIGHMEM as 'depends on EXPERT', updating the
>   Kconfig description to point to the kernel summit discussion
>   any any decisions made here.
>
>2. Change the ARMv7 Kconfig defaults to CONFIG_VMSPLIT_3G_OPT
>   and HIGHMEM=n, to make it more likely to find possible
>   regressions with that, without changing much for users.
>   Possibly do the same for x86 and powerpc.
>
>3. Start removing __GFP_HIGHMEM from in-kernel allocations
>   other than the page cache, in particular from individual
>   device drivers and filesystem metadata where there is
>   already little benefit.
>
>4. Remove HIGHMEM as an option from platforms that are thought
>   to no longer need it (arc, armv5, ppc4xx, ppc82xx, ppc83xx,
>   csky, microblaze, xtensa), mainly to validate the
>   assertion that these use only lowmem.
>
>5. Split highmem on zram out into a separate Kconfig option
>   that can be enabled without CONFIG_HIGHMEM or CONFIG_EXPERT
>
>6. Finish the "densemem" replacement for sparsemem, ideally
>   allowing both very sparse lowmem areas and a boot-time
>   vmsplit selection instead of the compile-time one.
>
>7. Finally, remove the highmem pagecache option, leaving only
>   zram and custom device drivers as a way to access high
>   pages.
>
>That last step should wait for an LTS kernel, ideally a version
>that the CIP project's SLTS kernel is planning to keep supporting
>for 10 years. The newest SLTS kernel was 6.12 last year, and the
>phb-crytal-ball suggests that the next one in December 2026 may
>be linux-7.4, the one after that in December 2028 (linux-7.15?)
>seems too far out to plan for but would be another option.
>
>Unless there is an easy consensus on this on the mailing list,
>I would like to lead a discussion session at the kernel summit
>in order to get closer to a decision.
>
>     Arnd 
>
>[1] https://osseu2025.sched.com/event/25VmZ/32-bit-linux-support-now-and-in-the-future-arnd-bergmann-linaro
>[2] https://www.youtube.com/watch?v=QiOMiyGCoTw
>[3] https://lwn.net/Articles/1035727/
>[4] https://lore.kernel.org/all/0047f565-ada4-491a-b157-f2d8dfde0ac0@app.fastmail.com/
>[5] https://lwn.net/Articles/813201/
>[6] https://lpc.events/event/16/contributions/1183/attachments/1062/2028/highmem-api-2022-09-12.pdf
>

1 GB systems used highmem too, sadly. And 1 GB was the norm for a big chuck of the late 32-bit era.