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SubjectRe: [PATCH v3 1/5] kernel/sysctl: support setting sysctl parameters from kernel command line
On Mon, 27 Apr 2020 20:04:29 +0200 Vlastimil Babka <vbabka@suse.cz> 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?

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