Re: [PATCH v3 3/4] arch, mm: consolidate empty_zero_page

Thomas Weißschuh
2026-04-16T11:20:21Z

On Thu, Apr 16, 2026 at 02:02:27PM +0300, Mike Rapoport wrote:
> On Thu, Apr 16, 2026 at 10:10:06AM +0200, Thomas Weißschuh wrote:
> > On Wed, Feb 11, 2026 at 12:31:40PM +0200, Mike Rapoport wrote:
> > > From: "Mike Rapoport (Microsoft)" <rppt@kernel.org>
> > >
> > > Reduce 22 declarations of empty_zero_page to 3 and 23 declarations of
> > > ZERO_PAGE() to 4.
> > >
> > > Every architecture defines empty_zero_page that way or another, but for the
> > > most of them it is always a page aligned page in BSS and most definitions
> > > of ZERO_PAGE do virt_to_page(empty_zero_page).
> > >
> > > Move Linus vetted x86 definition of empty_zero_page and ZERO_PAGE() to the
> > > core MM and drop these definitions in architectures that do not implement
> > > colored zero page (MIPS and s390).
> > >
> > > ZERO_PAGE() remains a macro because turning it to a wrapper for a static
> > > inline causes severe pain in header dependencies.
> > >
> > > For the most part the change is mechanical, with these being noteworthy:
> > >
> > > * alpha: aliased empty_zero_page with ZERO_PGE that was also used for boot
> > > parameters. Switching to a generic empty_zero_page removes the aliasing
> > > and keeps ZERO_PGE for boot parameters only
> > > * arm64: uses __pa_symbol() in ZERO_PAGE() so that definition of
> > > ZERO_PAGE() is kept intact.
> > > * m68k/parisc/um: allocated empty_zero_page from memblock,
> > > although they do not support zero page coloring and having it in BSS
> > > will work fine.
> > > * sparc64 can have empty_zero_page in BSS rather allocate it, but it
> > > can't use virt_to_page() for BSS. Keep it's definition of ZERO_PAGE()
> > > but instead of allocating it, make mem_map_zero point to
> > > empty_zero_page.
> > > * sh: used empty_zero_page for boot parameters at the very early boot.
> > > Rename the parameters page to boot_params_page and let sh use the generic
> > > empty_zero_page.
> >
> > With this in mainline as commit 6215d9f4470f ("arch, mm: consolidate
> > empty_zero_page") booting sh on QEMU is now broken.
> > The machine hangs before any output.
>
> Hmm, looks like sh does not like boot_param_page declared as unsigned char *
> This fixes the issue for me:
>
> diff --git a/arch/sh/include/asm/setup.h b/arch/sh/include/asm/setup.h
> index 63c9efc06348..b7c4469cb61e 100644
> --- a/arch/sh/include/asm/setup.h
> +++ b/arch/sh/include/asm/setup.h
> @@ -3,12 +3,13 @@
> #define _SH_SETUP_H
>
> #include <uapi/asm/setup.h>
> +#include <asm/page.h>
>
> /*
> * This is set up by the setup-routine at boot-time
> */
> -extern unsigned char *boot_params_page;
> -#define PARAM boot_params_page
> +extern unsigned long boot_params_page[PAGE_SIZE / sizeof(unsigned long)];
> +#define PARAM ((unsigned char *)boot_params_page)
>
> #define MOUNT_ROOT_RDONLY (*(unsigned long *) (PARAM+0x000))
> #define RAMDISK_FLAGS (*(unsigned long *) (PARAM+0x004))

Seems weird but works.

Tested-by: Thomas Weißschuh <thomas.weissschuh@linutronix.de>

Thanks!

> > Reproducer:
> > ./tools/testing/kunit/kunit.py run --arch sh --cross_compile sh4-linux- --raw_output=all example
> >
> > > * hexagon: had an amusing comment about empty_zero_page
> > >
> > > /* A handy thing to have if one has the RAM. Declared in head.S */
> > >
> > > that unfortunately had to go :)
> >
> > (...)