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