Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Tue, 28 Apr 2020 12:18:24 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        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>,
        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>
Subject: Re: [PATCH v3 1/5] kernel/sysctl: support setting sysctl parameters
 from kernel command line
Message-ID: <20200428101824.GM28637@dhcp22.suse.cz>
References: <20200427180433.7029-1-vbabka@suse.cz>
 <20200427180433.7029-2-vbabka@suse.cz>
 <20200427113331.f0c1e8e7cee98644260448d3@linux-foundation.org>
 <d7d01e9e-6b6d-47ce-c750-cd7296e30613@suse.cz>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <d7d01e9e-6b6d-47ce-c750-cd7296e30613@suse.cz>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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@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?

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