[TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

[Andreas Larsson]

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