HIGHMEM freeing patch breaks Realtek RTL930x builds

[<markus.stockhausen@gmx.de>]

Hi,

sorry for being late on this topic but downstream OpenWrt just started
kernel
conversion from 6.12 to 6.18 these days. During preparation of the PR
https://github.com/openwrt/openwrt/pull/21181 we noticed that Realtek 
RTL930x soc based devices with more than 256MB (highmem) do not boot 
any longer. 

These are MIPS 34k 32bit multithreaded SoC with layout 
<0x00000000 0x10000000>, /* 256 MiB lowmem */
<0x20000000 0x10000000>; /* 256 MiB highmem */

Bisecting the issue gave " arch, mm: streamline HIGHMEM freeing" 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/
?h=v6.15-rc1&id=6faea3422e3b4e8de44a55aa3e6e843320da66d2
as the first bad commit. This is back from the 6.15 times.

I have no real idea why removing mem_init_free_highmem() and letting
__free_memory_core() work on the whole memory range gives issues.

We are aligning to upstream very slowly and are still in need of 
downstream patches so here some additional info.

- Until now we never cared about FLATMEM/SPARSEMEM configs

- We are still using dedicated prom.c/setup.c for the devices
https://git.openwrt.org/?p=openwrt/openwrt.git;a=tree;f=target/linux/
realtek/files-6.12/arch/mips/rtl838x;hb=HEAD

Any idea or hint is appreciated.

Thanks in advance.

Markus