linux-kernel - Re: [PATCH mm-unstable] arch, mm: consolidate empty_zero

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Message-ID: <2157220c-0394-40fa-9918-a8514171bd10@gaisler.com>
Date: Tue, 27 Jan 2026 17:02:39 +0100
From: Andreas Larsson <andreas@...sler.com>
To: Mike Rapoport <rppt@...nel.org>, Andrew Morton <akpm@...ux-foundation.org>
Cc: Borislav Petkov <bp@...en8.de>, Brian Cain <bcain@...nel.org>,
 Catalin Marinas <catalin.marinas@....com>,
 "David S. Miller" <davem@...emloft.net>,
 Dave Hansen <dave.hansen@...ux.intel.com>,
 David Hildenbrand <david@...nel.org>, Dinh Nguyen <dinguyen@...nel.org>,
 Geert Uytterhoeven <geert@...ux-m68k.org>, Guo Ren <guoren@...nel.org>,
 Helge Deller <deller@....de>, Huacai Chen <chenhuacai@...nel.org>,
 Ingo Molnar <mingo@...hat.com>, Johannes Berg <johannes@...solutions.net>,
 John Paul Adrian Glaubitz <glaubitz@...sik.fu-berlin.de>,
 "Liam R. Howlett" <Liam.Howlett@...cle.com>,
 Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Madhavan Srinivasan <maddy@...ux.ibm.com>,
 Magnus Lindholm <linmag7@...il.com>, Matt Turner <mattst88@...il.com>,
 Max Filippov <jcmvbkbc@...il.com>, Michael Ellerman <mpe@...erman.id.au>,
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 Russell King <linux@...linux.org.uk>, Stafford Horne <shorne@...il.com>,
 Suren Baghdasaryan <surenb@...gle.com>, Thomas Gleixner <tglx@...nel.org>,
 Vineet Gupta <vgupta@...nel.org>, Vlastimil Babka <vbabka@...e.cz>,
 Will Deacon <will@...nel.org>, linux-alpha@...r.kernel.org,
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 linux-m68k@...ts.linux-m68k.org, linux-openrisc@...r.kernel.org,
 linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
 linux-riscv@...ts.infradead.org, linux-sh@...r.kernel.org,
 sparclinux@...r.kernel.org, linux-um@...ts.infradead.org,
 linux-mm@...ck.org, x86@...nel.org
Subject: Re: [PATCH mm-unstable] arch, mm: consolidate empty_zero_page

On 2026-01-24 10:56, Mike Rapoport wrote:
> From: "Mike Rapoport (Microsoft)" <rppt@...nel.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/sparc64/um: allocated empty_zero_page from memblock,
>   although they do not support zero page coloring and having it in BSS
>   will work fine.
> * 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.
> * 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 :)
> 
> Signed-off-by: Mike Rapoport (Microsoft) <rppt@...nel.org>
> ---

Running this in an LDOM on an UltraSparc T4 sparc64, the entire LDOM
hangs after a while during boot.

> diff --git a/arch/sparc/mm/init_64.c b/arch/sparc/mm/init_64.c
> index c2d19c9a9244..2bd99944176d 100644
> --- a/arch/sparc/mm/init_64.c
> +++ b/arch/sparc/mm/init_64.c
> @@ -177,9 +177,6 @@ extern unsigned long sparc_ramdisk_image64;
>  extern unsigned int sparc_ramdisk_image;
>  extern unsigned int sparc_ramdisk_size;
>  
> -struct page *mem_map_zero __read_mostly;
> -EXPORT_SYMBOL(mem_map_zero);
> -
>  unsigned int sparc64_highest_unlocked_tlb_ent __read_mostly;
>  
>  unsigned long sparc64_kern_pri_context __read_mostly;
> @@ -2506,18 +2503,6 @@ void __init mem_init(void)
>  	 */
>  	register_page_bootmem_info();
>  
> -	/*
> -	 * Set up the zero page, mark it reserved, so that page count
> -	 * is not manipulated when freeing the page from user ptes.
> -	 */
> -	mem_map_zero = alloc_pages(GFP_KERNEL|__GFP_ZERO, 0);
> -	if (mem_map_zero == NULL) {
> -		prom_printf("paging_init: Cannot alloc zero page.\n");
> -		prom_halt();
> -	}
> -	mark_page_reserved(mem_map_zero);
> -
> -
>  	if (tlb_type == cheetah || tlb_type == cheetah_plus)
>  		cheetah_ecache_flush_init();
>  }

This just removes the mark_page_reserved(mem_map_zero) without 
replacing it with something corresponding to that. Perhaps part
of the problem?

Cheers,
Andreas