Re: HIGHMEM freeing patch breaks Realtek RTL930x builds

[Mike Rapoport <rppt@kernel.org>]

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?

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

diff --git a/arch/mips/mm/init.c b/arch/mips/mm/init.c
index cd04200d0573..be5985155509 100644
--- a/arch/mips/mm/init.c
+++ b/arch/mips/mm/init.c
@@ -419,6 +419,23 @@ void __init arch_zone_limits_init(unsigned long *max_zone_pfns)
 static struct kcore_list kcore_kseg0;
 #endif
 
+static inline void __init mem_init_free_highmem(void)
+{
+#ifdef CONFIG_HIGHMEM
+	unsigned long tmp;
+
+	if (cpu_has_dc_aliases)
+		return;
+
+	for (tmp = highstart_pfn; tmp < highend_pfn; tmp++) {
+		struct page *page = pfn_to_page(tmp);
+
+		if (!memblock_is_memory(PFN_PHYS(tmp)))
+			SetPageReserved(page);
+	}
+#endif
+}
+
 void __init arch_mm_preinit(void)
 {
 	/*
@@ -429,6 +446,7 @@ void __init arch_mm_preinit(void)
 
 	maar_init();
 	setup_zero_pages();	/* Setup zeroed pages.  */
+	mem_init_free_highmem();
 
 #ifdef CONFIG_64BIT
 	if ((unsigned long) &_text > (unsigned long) CKSEG0)
 
> Thanks in advance.
> Markus

-- 
Sincerely yours,
Mike.