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

[Vlastimil Babka]

On 4/27/20 8:33 PM, Andrew Morton wrote:
> On Mon, 27 Apr 2020 20:04:29 +0200 Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> > ...
> >  
> > +	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?

AFAIK the "." is traditional, and "/" is a newer artefact of moving from the 
binary syscall form to procfs based form. So by "command-line syntax" you mean 
echo and cat, not sysctl tool? Because "man sysctl" says:

variable
	The name of a key to read from.  An example is kernel.ostype.  The '/' 
separator is also accepted in place of a '.'.

So I'm not strongly against supporting only / but I expect most people are used 
to the . and it will take them two attempts to pass the sysctl boot parameter 
correctly if they don't use it regularly - first trying . form, wonder why it 
doesn't work, then read the doc and realize it's not supported?

Vlastimil