[TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

[H. Peter Anvin]

On September 10, 2025 10:38:15 PM PDT, Andreas Larsson <andreas at gaisler.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.