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

["David Hildenbrand (arm)" <david@kernel.org>]

On 1/24/26 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.

That's just what I wanted to ask after looking at it :)

> 
> 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>

[...]

>   
> +#ifndef __HAVE_COLOR_ZERO_PAGE
> +extern unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)];
> +#endif
> +
> +#ifndef ZERO_PAGE
> +#define ZERO_PAGE(vaddr) ((void)(vaddr),virt_to_page(empty_zero_page))

Took me a second ...

> +#endif
> +
>   #endif /* !__ASSEMBLY__ */
>   
>   #if !defined(MAX_POSSIBLE_PHYSMEM_BITS) && !defined(CONFIG_64BIT)
> diff --git a/mm/mm_init.c b/mm/mm_init.c
> index 1a29a719af58..8ea5b76f317f 100644
> --- a/mm/mm_init.c
> +++ b/mm/mm_init.c
> @@ -53,6 +53,15 @@ EXPORT_SYMBOL(mem_map);
>   void *high_memory;
>   EXPORT_SYMBOL(high_memory);
>   
> +#ifndef __HAVE_COLOR_ZERO_PAGE
> +/*
> + * ZERO_PAGE is a global shared page that is always zero: used
> + * for zero-mapped memory areas etc..
> + */

I am not so sure of how much value that comment here is.

... and in particular, why it is not next to ZERO_PAGE in pgtable.h? ;)

> +unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;

Is there a good (or historic) reason why this is not simply

uint8_t empty_zero_page[PAGE_SIZE] __page_aligned_bss;

Almost looks to simple, right? So there must be some problem with it :)

> +EXPORT_SYMBOL(empty_zero_page);
> +#endif

Nothing else jumped at me, nice cleanup ... if you get sparc to work :P

-- 
Cheers,

David