Re: HIGHMEM freeing patch breaks Realtek RTL930x builds

[<markus.stockhausen@gmx.de>]

Hi Mike, 

> Von: Mike Rapoport <rppt@kernel.org> 
> Gesendet: Sonntag, 28. Dezember 2025 10:54
> Betreff: Re: HIGHMEM freeing patch breaks Realtek RTL930x builds
> 
> Hi Markus,
>
> On Sat, Dec 20, 2025 at 09:57:40AM +0100, markus.stockhausen@gmx.de wrote:
> > 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.
>
> Can you please send logs from a working kernel and a failing kernel with
> "memblock=debug" added to the kernel command line?

Good hint. I've done that and collected all information in
https://github.com/openwrt/openwrt/issues/21323

> Another thing I think worth checking is will the system boot with a
partial
> revert of 6faea3422e3b ("arch, mm: streamline HIGHMEM freeing") for mips:

Will try this out next, update the issue and inform you here.

Thank you

Markus