Re: [arnd-playground:highmem-7.4 5/6] arch/powerpc/mm/init_32.c:49:2: error: #error "You must adjust CONFIG_LOWMEM_SIZE or CONFIG_KERNEL_START"

["Arnd Bergmann" <arnd@arndb.de>]

On Thu, Nov 12, 2026, at 17:13, kernel test robot wrote:
>
> If you fix the issue in a separate patch/commit (i.e. not just a new 
> version of
> the same patch/commit), kindly add following tags
> | Reported-by: kernel test robot <lkp@intel.com>
> | Closes: 
> https://lore.kernel.org/oe-kbuild-all/202611121726.xboRqpix-lkp@intel.com/
>
> All errors (new ones prefixed by >>):
>
>>> arch/powerpc/mm/init_32.c:49:2: error: #error "You must adjust CONFIG_LOWMEM_SIZE or CONFIG_KERNEL_START"
>     #error "You must adjust CONFIG_LOWMEM_SIZE or CONFIG_KERNEL_START"
>      ^~~~~

The randconfig file contains

# CONFIG_LOWMEM_SIZE_BOOL is not set
CONFIG_LOWMEM_SIZE=0x80000000
CONFIG_PAGE_OFFSET_BOOL=y
CONFIG_PAGE_OFFSET=0xc0000000
CONFIG_KERNEL_START_BOOL=y
CONFIG_KERNEL_START=0xc0000000

which is an invalid configuration

My patch changed the defaults for both PAGE_OFFSET and LOWMEM_SIZE
in a way that should still work:

 config LOWMEM_SIZE
        hex "Maximum low memory size (in bytes)" if LOWMEM_SIZE_BOOL
-       default "0x30000000"
+       default "0x80000000" if HIGHMEM
+       default "0x40000000"
 
 config PAGE_OFFSET
        hex "Virtual address of memory base" if PAGE_OFFSET_BOOL
-       default "0xc0000000"
+       default "0x70000000" if HIGHMEM
+       default "0xb0000000"

What I'm guessing happened is that the .config file was generated
with the old defaults by running 'make randconfig' on an unpatched
tree, but then running 'make olddefconfig' changes the default
for LOWMEM_SIZE without changing the others.

     Arnd