linux-kernel - Re: [PATCH V5 1/2] arm64: refactor the rodata=xxx

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Message-ID: <cc2207e4-2118-41be-bd35-97cbdbe85521@infradead.org>
Date: Sun, 29 Jun 2025 20:25:36 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Huang Shijie <shijie@...amperecomputing.com>, catalin.marinas@....com,
 will@...nel.org, corbet@....net
Cc: patches@...erecomputing.com, cl@...ux.com, yang@...amperecomputing.com,
 akpm@...ux-foundation.org, paulmck@...nel.org, rostedt@...dmis.org,
 Neeraj.Upadhyay@....com, bp@...en8.de, ardb@...nel.org,
 anshuman.khandual@....com, suzuki.poulose@....com, gshan@...hat.com,
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH V5 1/2] arm64: refactor the rodata=xxx



On 6/29/25 8:02 PM, Huang Shijie wrote:
> As per admin guide documentation, "rodata=on" should be the default on
> platforms. Documentation/admin-guide/kernel-parameters.txt describes
> these options as
> 
>    rodata=         [KNL,EARLY]
>            on      Mark read-only kernel memory as read-only (default).
>            off     Leave read-only kernel memory writable for debugging.
>            full    Mark read-only kernel memory and aliases as read-only
>                    [arm64]
> 
> But on arm64 platform, "rodata=full" is the default instead. This patch
> implements the following changes.
> 
>  - Make "rodata=on" behaviour same as the original "rodata=full"
>  - Make "rodata=noalias" (new) behaviour same as the original "rodata=on"
>  - Drop the original "rodata=full"
>  - Add comment for arch_parse_debug_rodata()
>  - Update kernel-parameters.txt as required
> 
> After this patch, the "rodata=on" will be the default on arm64 platform
> as well.
> 
> Signed-off-by: Huang Shijie <shijie@...amperecomputing.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  2 +-
>  arch/arm64/include/asm/setup.h                | 28 +++++++++++++++++--
>  2 files changed, 27 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ee0735c6b8e2..e0cd6dac26d3 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6354,7 +6354,7 @@
>  	rodata=		[KNL,EARLY]
>  		on	Mark read-only kernel memory as read-only (default).
>  		off	Leave read-only kernel memory writable for debugging.
> -		full	Mark read-only kernel memory and aliases as read-only
> +		noalias	Use more block mappings,may have better performance.

Add space after comma, please.                 ^

>  		        [arm64]
>  
>  	rockchip.usb_uart
> diff --git a/arch/arm64/include/asm/setup.h b/arch/arm64/include/asm/setup.h
> index ba269a7a3201..6b994d0881d1 100644
> --- a/arch/arm64/include/asm/setup.h
> +++ b/arch/arm64/include/asm/setup.h
> @@ -13,6 +13,30 @@
>  extern phys_addr_t __fdt_pointer __initdata;
>  extern u64 __cacheline_aligned boot_args[4];
>  
> +/*
> + * rodata=on (default)
> + *
> + *    This applies read-only attributes to VM areas and to the linear
> + *    alias of the backing pages as well. This prevents code or read-
> + *    only data from being modified (inadvertently or intentionally),
> + *    via another mapping for the same memory page.
> + *
> + *    But this might cause linear map region to be mapped down to base
> + *    pages, which may adversely affect performance in some cases.
> + *
> + * rodata=off
> + *
> + *    This provides more block mappings and contiguous hints for linear
> + *    map region which would minimize TLB footprint. This also leaves
> + *    read-only kernel memory writable for debugging.
> + *
> + * rodata=noalias
> + *
> + *    This provides more block mappings and contiguous hints for linear
> + *    map region which would minimize TLB footprint. This leaves the linear
> + *    alias of read-only mappings in the vmalloc space writeable, making
> + *    them susceptible to inadvertent modification by software.
> + */
>  static inline bool arch_parse_debug_rodata(char *arg)
>  {
>  	extern bool rodata_enabled;
> @@ -21,7 +45,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
>  	if (!arg)
>  		return false;
>  
> -	if (!strcmp(arg, "full")) {
> +	if (!strcmp(arg, "on")) {
>  		rodata_enabled = rodata_full = true;
>  		return true;
>  	}
> @@ -31,7 +55,7 @@ static inline bool arch_parse_debug_rodata(char *arg)
>  		return true;
>  	}
>  
> -	if (!strcmp(arg, "on")) {
> +	if (!strcmp(arg, "noalias")) {
>  		rodata_enabled = true;
>  		rodata_full = false;
>  		return true;

-- 
~Randy