Re: [PATCH 10/13] arch, mm: set high_memory in free_area_init()

From: "Russell King (Oracle)" <linux@armlinux.org.uk>
To: Mark Brown <broonie@kernel.org>
Cc: Mike Rapoport <rppt@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Alexander Gordeev <agordeev@linux.ibm.com>,
	Andreas Larsson <andreas@gaisler.com>,
	Andy Lutomirski <luto@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Borislav Petkov <bp@alien8.de>, Brian Cain <bcain@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	"David S. Miller" <davem@davemloft.net>,
	Dinh Nguyen <dinguyen@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Gerald Schaefer <gerald.schaefer@linux.ibm.com>,
	Guo Ren <guoren@kernel.org>, Heiko Carstens <hca@linux.ibm.com>,
	Helge Deller <deller@gmx.de>, Huacai Chen <chenhuacai@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>,
	Johannes Berg <johannes@sipsolutions.net>,
	John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>,
	Madhavan Srinivasan <maddy@linux.ibm.com>,
	Matt Turner <mattst88@gmail.com>,
	Max Filippov <jcmvbkbc@gmail.com>,
	Michael Ellerman <mpe@ellerman.id.au>,
	Michal Simek <monstr@monstr.eu>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Richard Weinberger <richard@nod.at>,
	Stafford Horne <shorne@gmail.com>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Vasily Gorbik <gor@linux.ibm.com>,
	Vineet Gupta <vgupta@kernel.org>, Will Deacon <will@kernel.org>,
	linux-alpha@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-snps-arc@lists.infradead.org,
	linux-arm-kernel@lists.infradead.org, linux-csky@vger.kernel.org,
	linux-hexagon@vger.kernel.org, loongarch@lists.linux.dev,
	linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org,
	linux-openrisc@vger.kernel.org, linux-parisc@vger.kernel.org,
	linuxppc-dev@lists.ozlabs.org, linux-riscv@lists.infradead.org,
	linux-s390@vger.kernel.org, linux-sh@vger.kernel.org,
	sparclinux@vger.kernel.org, linux-um@lists.infradead.org,
	linux-arch@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org
Subject: Re: [PATCH 10/13] arch, mm: set high_memory in free_area_init()
Date: Tue, 11 Mar 2025 22:05:25 +0000	[thread overview]
Message-ID: <Z9CzpTlA2e0jR7UJ@shell.armlinux.org.uk> (raw)
In-Reply-To: <5e40219b-f149-4e0f-aa10-c09fa183945e@sirena.org.uk>

On Tue, Mar 11, 2025 at 09:33:29PM +0000, Mark Brown wrote:
> [    0.000000] Booting Linux on physical CPU 0x0
> [    0.000000] Linux version 6.14.0-rc6-next-20250311 (tuxmake@tuxmake) (arm-linux-gnueabihf-gcc (Debian 13.3.0-5) 13.3.0, GNU ld (GNU Binutils for Debian) 2.43.1) #1 SMP @1741691801
> [    0.000000] CPU: ARMv7 Processor [414fc0f0] revision 0 (ARMv7), cr=10c5387d
> [    0.000000] CPU: div instructions available: patching division code
> [    0.000000] CPU: PIPT / VIPT nonaliasing data cache, PIPT instruction cache
> [    0.000000] OF: fdt: Machine model: linux,dummy-virt
> [    0.000000] random: crng init done
> [    0.000000] earlycon: pl11 at MMIO 0x09000000 (options '')
> [    0.000000] printk: legacy bootconsole [pl11] enabled
> [    0.000000] Memory policy: Data cache writealloc
> [    0.000000] efi: UEFI not found.
> [    0.000000] cma: Reserved 64 MiB at 0x00000000

If that CMA address is correct, then it's wrong. virt machines start
DRAM at 0x40000000. This is a small memory VM:

[    0.000000] Zone ranges:
[    0.000000]   Normal   [mem 0x0000000040000000-0x0000000045ffffff]
[    0.000000]   HighMem  empty

and this is a larger memory VM:

[    0.000000] Zone ranges:
[    0.000000]   Normal   [mem 0x0000000040000000-0x000000006fffffff]
[    0.000000]   HighMem  [mem 0x0000000070000000-0x000000007fffffff]

Neither have CMA enabled (it's not necessary for a VM).

On a real platform where CMA and highmem is enabled, then:

[    0.000000] cma: Reserved 16 MiB at 0x4f000000 on node -1
[    0.000000] Zone ranges:
[    0.000000]   Normal   [mem 0x0000000010000000-0x000000003fffffff]
[    0.000000]   HighMem  [mem 0x0000000040000000-0x000000004fffffff]

So that "cma:" line you are seeing is indicating that something is very
very wrong - it should definitely not be zero.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
FTTP is here! 80Mbps down 10Mbps up. Decent connectivity at last!