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Subject: Re: [PATCH v3 1/5] kernel/sysctl: support setting sysctl parameters
 from kernel command line
To:     Andrew Morton <akpm@linux-foundation.org>
Cc:     Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        linux-kernel@vger.kernel.org, linux-api@vger.kernel.org,
        linux-mm@kvack.org, Ivan Teterevkov <ivan.teterevkov@nutanix.com>,
        Michal Hocko <mhocko@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        "Guilherme G . Piccoli" <gpiccoli@canonical.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Michal Hocko <mhocko@suse.com>
References: <20200427180433.7029-1-vbabka@suse.cz>
 <20200427180433.7029-2-vbabka@suse.cz>
 <20200427113331.f0c1e8e7cee98644260448d3@linux-foundation.org>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <d7d01e9e-6b6d-47ce-c750-cd7296e30613@suse.cz>
Date:   Tue, 28 Apr 2020 10:09:37 +0200
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On 4/27/20 8:33 PM, Andrew Morton wrote:
> On Mon, 27 Apr 2020 20:04:29 +0200 Vlastimil Babka <vbabka@suse.cz> 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