[TECH TOPIC] Reaching consensus on CONFIG_HIGHMEM phaseout

[Nicolas Schichan]

> On Fri, Sep 19, 2025, at 16:34, Jason Gunthorpe wrote:
> > On Fri, Sep 19, 2025 at 04:22:20PM +0200, Arnd Bergmann wrote:
[...]
> > I am right it sounds like we can drop vivt cache support with highmem?
> > vivt is alot easier to understand if kmap isn't running around
> > creating virtual addresses aliases!
>
> I think so, yes. I've prototyped a patch for it to add a dependency
> on !VIVT and the drop the code paths that are clearly no longer
> needed without it. This obviously needs proper testing, but the
> diffstat is promising:
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index f14bac913d50..0a31cfaf3818 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1214,6 +1214,7 @@ config ARCH_SPARSEMEM_ENABLE
>  config HIGHMEM
>         bool "High Memory Support"
>         depends on MMU
> +       depends on !CPU_CACHE_VIVT
>         select KMAP_LOCAL
>         select KMAP_LOCAL_NON_LINEAR_PTE_ARRAY
>         help
>  arch/arm/Kconfig                    |  1 +
>  arch/arm/configs/gemini_defconfig   |  1 -
>  arch/arm/configs/multi_v5_defconfig |  1 -
>  arch/arm/configs/mvebu_v5_defconfig |  1 -
>  arch/arm/include/asm/highmem.h      | 56 +-----------------------
>  arch/arm/mm/cache-feroceon-l2.c     | 31 +------------
>  arch/arm/mm/cache-xsc3l2.c          | 47 +++-----------------
>  arch/arm/mm/dma-mapping.c           | 14 ++----
>  arch/arm/mm/flush.c                 | 19 ++------
>  mm/highmem.c                        | 86 +++++--------------------------------
>  10 files changed, 27 insertions(+), 230 deletions(-)
>
> I did get an email from Nicolas Schichan (added to Cc here),
> and he is still supporting a widely deployed Kirkwood based
> platform that uses 1GB RAM configurations. He should get
> a chance to test that with CONFIG_VMSPLIT_3G_OPT, but I
> would expect that to continue working, possibly with minor
> bugfixes.

Hello Arnd,

We don't use HIGMEM on our Kirkwood platform, we are happy using a
2G/2G WMSPLIT. We don't need a lot of virtual address space for
userland, and with the 2G split we don't waste physical memory.

I'm happy to test your patch serie with VMSPLIT_3G_OPT and see if it
still boots with it once you send it.

Regards,

-- 
Nicolas Schichan