linux-kernel - Re: [PATCH v2 04/10] arm64: mm: Preserve existing table mappings when mapping DRAM

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Message-ID: <487034a1-817b-4fc4-a1ff-3ab028dc80bf@arm.com>
Date: Tue, 27 Jan 2026 09:58:27 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Ard Biesheuvel <ardb+git@...gle.com>, linux-kernel@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org, will@...nel.org,
 catalin.marinas@....com, mark.rutland@....com,
 Ard Biesheuvel <ardb@...nel.org>,
 Anshuman Khandual <anshuman.khandual@....com>,
 Liz Prucka <lizprucka@...gle.com>, Seth Jenkins <sethjenkins@...gle.com>,
 Kees Cook <kees@...nel.org>, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2 04/10] arm64: mm: Preserve existing table mappings when
 mapping DRAM

On 26/01/2026 09:26, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> Instead of blindly overwriting an existing table entry when mapping DRAM
> regions, take care not to replace a pre-existing table entry with a
> block entry. This permits the logic of mapping the kernel's linear alias
> to be simplified in a subsequent patch.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>

Reviewed-by: Ryan Roberts <ryan.roberts@....com>

> ---
>  arch/arm64/mm/mmu.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index 9d39de3cfe67..28cc3cda042c 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -262,7 +262,8 @@ static int init_pmd(pmd_t *pmdp, unsigned long addr, unsigned long end,
>  
>  		/* try section mapping first */
>  		if (((addr | next | phys) & ~PMD_MASK) == 0 &&
> -		    (flags & NO_BLOCK_MAPPINGS) == 0) {
> +		    (flags & NO_BLOCK_MAPPINGS) == 0 &&
> +		    !pmd_table(old_pmd)) {
>  			pmd_set_huge(pmdp, phys, prot);
>  
>  			/*
> @@ -385,7 +386,8 @@ static int alloc_init_pud(p4d_t *p4dp, unsigned long addr, unsigned long end,
>  		 */
>  		if (pud_sect_supported() &&
>  		   ((addr | next | phys) & ~PUD_MASK) == 0 &&
> -		    (flags & NO_BLOCK_MAPPINGS) == 0) {
> +		    (flags & NO_BLOCK_MAPPINGS) == 0 &&
> +		    !pud_table(old_pud)) {
>  			pud_set_huge(pudp, phys, prot);
>  
>  			/*