Re: [PATCH v3 01/13] arm64: Move the zero page to rodata

Ard Biesheuvel
2026-04-08T15:09:49Z

On Wed, 8 Apr 2026, at 15:43, Catalin Marinas wrote:
> Hi Ard,
>
> On Fri, Mar 20, 2026 at 03:59:36PM +0100, Ard Biesheuvel wrote:
>> diff --git a/arch/arm64/kernel/vmlinux.lds.S b/arch/arm64/kernel/vmlinux.lds.S
>> index 2964aad0362e..2d021a576e50 100644
>> --- a/arch/arm64/kernel/vmlinux.lds.S
>> +++ b/arch/arm64/kernel/vmlinux.lds.S
>> @@ -229,6 +229,7 @@ SECTIONS
>> #endif
>>
>> reserved_pg_dir = .;
>> + empty_zero_page = .;
>> . += PAGE_SIZE;
>>
>> swapper_pg_dir = .;
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index a6a00accf4f9..795743913ce5 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -66,9 +66,8 @@ long __section(".mmuoff.data.write") __early_cpu_boot_status;
>>
>> /*
>> * Empty_zero_page is a special page that is used for zero-initialized data
>> - * and COW.
>> + * and COW. Defined in the linker script.
>> */
>> -unsigned long empty_zero_page[PAGE_SIZE / sizeof(unsigned long)] __page_aligned_bss;
>> EXPORT_SYMBOL(empty_zero_page);
>
> I looked at Sashiko's reports
> (https://sashiko.dev/#/patchset/20260320145934.2349881-15-ardb+git@google.com)
> and it has a point here that with MTE, map_mem() doesn't map the
> empty_zero_page as Tagged in the for_each_mem_range() loop. The
> subsequent cpu_enable_mte() will fail to initialise the tags. I think
> this problem disappears with patch 11 where all the linear map is now
> Tagged.
>
> We either ignore it or we temporarily map the kernel as Tagged until the
> linear alias is removed later:
>
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 795743913ce5..5290f7537074 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1175,7 +1175,7 @@ static void __init map_mem(pgd_t *pgdp)
> * so we should avoid them here.
> */
> __map_memblock(pgdp, kernel_start, kernel_end,
> - PAGE_KERNEL, NO_CONT_MAPPINGS);
> + pgprot_tagged(PAGE_KERNEL), NO_CONT_MAPPINGS);
> memblock_clear_nomap(kernel_start, kernel_end - kernel_start);
> arm64_kfence_map_pool(early_kfence_pool, pgdp);
> }
>

OK. There were some other very good comments too, I'll look into those and respin this for the next cycle.