On September 9, 2025 2:23:37 PM PDT, Arnd Bergmann <a...@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-f2d8dfde0...@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.
