Re: [PATCH v3 1/5] kernel/sysctl: support setting sysctl parameters from kernel command line

[Andrew Morton]

On Mon, 27 Apr 2020 20:04:29 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:

> A recently proposed patch to add vm_swappiness command line parameter in
> addition to existing sysctl [1] made me wonder why we don't have a general
> support for passing sysctl parameters via command line. Googling found only
> somebody else wondering the same [2], but I haven't found any prior discussion
> with reasons why not to do this.
> 
> Settings the vm_swappiness issue aside (the underlying issue might be solved in
> a different way), quick search of kernel-parameters.txt shows there are already
> some that exist as both sysctl and kernel parameter - hung_task_panic,
> nmi_watchdog, numa_zonelist_order, traceoff_on_warning. A general mechanism
> would remove the need to add more of those one-offs and might be handy in
> situations where configuration by e.g. /etc/sysctl.d/ is impractical.
> 
> Hence, this patch adds a new parse_args() pass that looks for parameters
> prefixed by 'sysctl.' and tries to interpret them as writes to the
> corresponding sys/ files using an temporary in-kernel procfs mount. This
> mechanism was suggested by Eric W. Biederman [3], as it handles all dynamically
> registered sysctl tables, even though we don't handle modular sysctls. Errors
> due to e.g. invalid parameter name or value are reported in the kernel log.
> 
> The processing is hooked right before the init process is loaded, as some
> handlers might be more complicated than simple setters and might need some
> subsystems to be initialized. At the moment the init process can be started and
> eventually execute a process writing to /proc/sys/ then it should be also fine
> to do that from the kernel.
> 
> Sysctls registered later on module load time are not set by this mechanism -
> it's expected that in such scenarios, setting sysctl values from userspace is
> practical enough.
> 
> ...
>  
> +	sysctl.*=	[KNL]
> +			Set a sysctl parameter, right before loading the init
> +			process, as if the value was written to the respective
> +			/proc/sys/... file. Both '.' and '/' are recognized as
> +			separators. Unrecognized parameters and invalid values
> +			are reported in the kernel log. Sysctls registered
> +			later by a loaded module cannot be set this way.
> +			Example: sysctl.vm.swappiness=40

Why support "."?  I think only supporting "/" is perfectly adequate and
simplifies documentation.  It aligns the command-line syntax with the
rest of the sysctl documentation.  I'm not seeing the need to provide
two ways of doing the same thing?