Re: HIGHMEM freeing patch breaks Realtek RTL930x builds

[Mike Rapoport <rppt@kernel.org>]

On Mon, Dec 29, 2025 at 07:36:52PM +0100, markus.stockhausen@gmx.de wrote:
> 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

The successful boot disables highmem:
[    0.332305] Memory: 495640K/524288K available (8188K kernel code, 647K rwdata, 1504K rodata, 9620K init, 244K bss, 27528K reserved, 0K cma-reserved, 0K highmem)

And the failing boot actually enables it:
[    0.332285] Memory: 495640K/524288K available (8188K kernel code, 647K rwdata, 1504K rodata, 9620K init, 244K bss, 27528K reserved, 0K cma-reserved, 262144K highmem)

so I believe that the partial revert should help.

Anther option is to simply disable CONFIG_HIGHMEM for that platform if it
anyway can't support highmem.
 
> > 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

-- 
Sincerely yours,
Mike.