[TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

[Christophe Leroy]

Hi Richard,

Le 10/09/2025 à 16:04, Richard Weinberger a écrit :
> Arnd,
> 
> ----- Ursprüngliche Mail -----
>> Von: "Arnd Bergmann" <arnd at arndb.de>
>> 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.
> 
> I am part of a team responsible for products based on various 32-bit SoCs,
> so I'm alarmed.
> These products, which include ARMv7 and PPC32 architectures with up to 2 GiB of RAM,
> are communication systems with five-figure deployments worldwide.
> 
> Removing high memory will have an impact on these systems.
> The oldest kernel version they run is 4.19 LTS, with upgrades to a more recent
> LTS release currently in progress.
> We typically upgrade the kernel every few years and will continue to support
> these systems for at least the next 10 years.
> 
> Even with a new memory split, which could utilize most of the available memory,
> I expect there to be issues with various applications and FPGA device drivers.

Can you tell which PPC32 model/family you are using ? Is it mpc85xx or 
and/or other variants ? Maybe we can look at keeping CONFIG_HIGHMEM or 
find alternatives for that subset of PPC32 only.

Could you also elaborate a bit on the kind of issues you forsee or fear 
with applications and FPGA device drivers.

FWIW I sent out today a patch that removes CONFIG_HIGHMEM complely on 
powerpc in order to get a better view of the impacted areas and allow 
people to test what it looks like on their system without 
CONFIG_HIGHMEM. See [1].

Christophe

[1] 
https://patchwork.ozlabs.org/project/linuxppc-dev/patch/28d908b95fe358129db18f69b30891788e15ada0.1757512010.git.christophe.leroy@csgroup.eu/