Re: [PATCH mm-unstable] arch, mm: consolidate empty_zero_page

[Andreas Larsson <andreas@gaisler.com>]

On 2026-01-24 10:56, 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/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@kernel.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