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

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Message-ID: <20200428101824.GM28637@dhcp22.suse.cz>
Date:   Tue, 28 Apr 2020 12:18:24 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        linux-kernel@...r.kernel.org, linux-api@...r.kernel.org,
        linux-mm@...ck.org, Ivan Teterevkov <ivan.teterevkov@...anix.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        "Guilherme G . Piccoli" <gpiccoli@...onical.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH v3 1/5] kernel/sysctl: support setting sysctl parameters
 from kernel command line

On Tue 28-04-20 10:09:37, Vlastimil Babka wrote:
> On 4/27/20 8:33 PM, Andrew Morton wrote:
> > On Mon, 27 Apr 2020 20:04:29 +0200 Vlastimil Babka <vbabka@...e.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?

Yes, I do agree. I have only recently learned that sysctl supports / as
well. Most people are simply used to . notation. The copy of the arch
and . -> / substitution is a trivial operation and I do not think it is
a real reason to introduce unnecessarily harder to use interface.
-- 
Michal Hocko
SUSE Labs