linux-kernel - Re: [PATCH v2 06/10] arm64: mm: Remove bogus stop condition from map

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Message-ID: <005012a8-bfcc-4049-9ff2-adecf3a89c53@arm.com>
Date: Tue, 27 Jan 2026 10:06:25 +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 06/10] arm64: mm: Remove bogus stop condition from
 map_mem() loop

On 26/01/2026 09:26, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@...nel.org>
> 
> The memblock API guarantees that start is not greater than or equal to
> end, so there is no need to test it. And if were, it is doubtful that
> breaking out of the loop would be a reasonable course of action here
> (rather than attempting to map the remaining regions)
> 
> So let's drop this check.

If start were greater than end, I think it's a pretty safe bet that
__map_memblock() would do something horrible.

But if memblock makes the guarantee you claim I agree it's preferable to
declutter the code:

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

> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/arm64/mm/mmu.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index d7faa98f427c..377bdc4d84a1 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -1158,8 +1158,6 @@ static void __init map_mem(pgd_t *pgdp)
>  
>  	/* map all the memory banks */
>  	for_each_mem_range(i, &start, &end) {
> -		if (start >= end)
> -			break;
>  		/*
>  		 * The linear map must allow allocation tags reading/writing
>  		 * if MTE is present. Otherwise, it has the same attributes as