linux-kernel - Re: [PATCH v8 20/21] printk: Deprecate the kernel.printk sysctl interface

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Message-ID: <p5yuz76o3bln3brrr2bo3rtddpmv6woeroqqegjq4jrduoaj3p@ehbupxfrsnvq>
Date: Wed, 17 Dec 2025 12:47:42 +0100
From: Joel Granados <joel.granados@...nel.org>
To: Petr Mladek <pmladek@...e.com>
Cc: Chris Down <chris@...isdown.name>, linux-kernel@...r.kernel.org, 
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Sergey Senozhatsky <senozhatsky@...omium.org>, 
	Steven Rostedt <rostedt@...dmis.org>, John Ogness <john.ogness@...utronix.de>, 
	Geert Uytterhoeven <geert@...ux-m68k.org>, Tony Lindgren <tony.lindgren@...ux.intel.com>, 
	kernel-team@...com
Subject: Re: [PATCH v8 20/21] printk: Deprecate the kernel.printk sysctl
 interface

On Mon, Dec 15, 2025 at 05:06:40PM +0100, Petr Mladek wrote:
> On Mon 2025-12-15 10:52:28, Joel Granados wrote:
> > On Fri, Dec 12, 2025 at 04:51:46PM +0100, Petr Mladek wrote:
> > > Added Joel into Cc for the sysrq API changes.
> > Thx. I have just two comments (inline)
> > 
> > > 
> > > On Fri 2025-11-28 03:44:33, Chris Down wrote:
> > > > The kernel.printk sysctl interface is deprecated in favour of more
> > > > granular and clearer sysctl interfaces for controlling loglevels, namely
> > > > kernel.console_loglevel and kernel.default_message_loglevel.
> > > > 
> > > > - Encourage users to adopt the new, dedicated sysctl interfaces
> > > >   (kernel.console_loglevel and kernel.default_message_loglevel), which
> > > >   are more straightforward and easier to understand.
> > > > - Improve input validation and error handling, ensuring that users
> > > >   receive appropriate feedback when setting invalid values.
> > > > - Simplify the configuration of loglevels by exposing only the controls
> > > >   that are necessary and relevant.
> > > > 
> > > > --- a/kernel/printk/sysctl.c
> > > > +++ b/kernel/printk/sysctl.c
> > > > @@ -67,13 +68,24 @@ static int proc_dointvec_console_loglevel(const struct ctl_table *table,
> > > >  			       do_proc_dointvec_console_loglevel, NULL);
> > > >  }
> > > >  
> > > > +static int proc_dointvec_printk_deprecated(const struct ctl_table *table, int write,
> > > > +					   void *buffer, size_t *lenp, loff_t *ppos)
> > > > +{
> > > > +	int res = proc_dointvec(table, write, buffer, lenp, ppos);
> > > > +
> > > > +	if (write)
> > This would print out a warning when the user writes to the file.
> > I have some questions here:
> > 1. Is this sysctl to be completely removed in the future?
> 
> I hope so.
> 
> > 2. Shouldn't you warn on read as well (if it is going to be completely
> >    removed).
> 
> Makes perfect sense. We should.
> 
> > 3. Is there a plan for when this will be completely removed? Is there a
> >    date?
> 
> Honeslty, I do not have much experience with removing such API.
> I am not sure how aggressive we could be.
> 
> For example, it seems that it is going to take 15 years to obsolete
> automounting of tracefs in debugfs. But they added the warning
> 10 years after adding tracefs, see commit 9ba817fb7c6afd3
> ("tracing: Deprecate auto-mounting tracefs in debugfs").
Indeed, there is no agreed procedure. If you don't have an expected
date, then don't put it in the warn message.

> 
> > 4. Should you put that date in the message?
> 
> It would be useful. What about doing it in 10 years from now?
10 years seems ok to me. But since this is a guess more than a plan, I
would just not put it on the message. Up to you.

> 
> > > > +		pr_warn_once("printk: The kernel.printk sysctl is deprecated. Consider using kernel.console_loglevel or kernel.default_message_loglevel instead.\n");
> > Line seems very long. cut it in two?
> 
> OK, what about the following?
> 
> 	pr_warn_once("printk: The kernel.printk sysctl is deprecated and will be removed in 2036.\n");
> 	pr_warn_once("printk: Use kernel.console_loglevel or kernel.default_message_loglevel instead.\n");
I meant it more as a long line in the code. Not a long line in the
log. Like this:

  pr_warn_once("printk: The kernel.printk sysctl is deprecated. Instead, use "
               "kernel.console_loglevel or kernel.default_message_loglevel.\n");

Its still long, but more manageable.

Best

-- 

Joel Granados

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