linux-kernel - Re: [PATCH V2 5/5] panic: add note that panic

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Message-ID: <aFvBuOnD0cAEWJfl@U-2FWC9VHC-2323.local>
Date: Wed, 25 Jun 2025 17:30:32 +0800
From: Feng Tang <feng.tang@...ux.alibaba.com>
To: Petr Mladek <pmladek@...e.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Lance Yang <lance.yang@...ux.dev>, Jonathan Corbet <corbet@....net>,
	linux-kernel@...r.kernel.org, paulmck@...nel.org,
	john.ogness@...utronix.de
Subject: Re: [PATCH V2 5/5] panic: add note that panic_print interface is
 deprecated

On Mon, Jun 23, 2025 at 05:22:20PM +0200, Petr Mladek wrote:
> On Mon 2025-06-16 09:08:40, Feng Tang wrote:
> > Long term wise, the 'panic_sys_info' should be the only controlling
> > interface, which can be referred by other modules.
> 
> Strictly speaking, 'panic_sys_info' is not a complete
> replacement for 'panic_print' because it does not allow
> replaying the log buffer during panic.
> 
> I suggest to add a separate parameter "panic_console_replay"
> for the missing functionality first. And 'panic_print' will
> be obsoleted by both 'panic_sys_info' and 'panic_console_replay'.
> 
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -76,6 +76,13 @@ ATOMIC_NOTIFIER_HEAD(panic_notifier_list);
> >  EXPORT_SYMBOL(panic_notifier_list);
> >  
> >  #ifdef CONFIG_SYSCTL
> > +static int sysctl_panic_print_handler(const struct ctl_table *table, int write,
> > +			   void *buffer, size_t *lenp, loff_t *ppos)
> > +{
> > +	printk_once("panic: 'panic_print' sysctl interface will be obsoleted by 'panic_sys_info' interface.\n");
> > +	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
> > +}
> > +
> >  static const struct ctl_table kern_panic_table[] = {
> >  #ifdef CONFIG_SMP
> >  	{
> > @@ -107,7 +114,7 @@ static const struct ctl_table kern_panic_table[] = {
> >  		.data		= &panic_print,
> >  		.maxlen		= sizeof(unsigned long),
> >  		.mode		= 0644,
> > -		.proc_handler	= proc_doulongvec_minmax,
> > +		.proc_handler	= sysctl_panic_print_handler,
> 
> This handles only the sysctl interface. We should do something similar
> also for the "panic_print" command line parameter. It would require
> using core_param_cb() instead of core_param().

When testing the change, I found  a problem:  'core_param_cb' is not
the real counterpart of 'core_param', that it is a module parameter
instead of kernel/core parameter, and adds the module.prefix to the
parameter, say, the effective cmdline parameter is changed to
'panic.panic_print=' instead of 'panic_print='.

While below patch of 'kernel_param_cb' works fine, but I'm not sure if
it is worth the change:

---
diff --git a/include/linux/moduleparam.h b/include/linux/moduleparam.h
index bfb85fd13e1f..71053d078cea 100644
--- a/include/linux/moduleparam.h
+++ b/include/linux/moduleparam.h
@@ -194,6 +194,9 @@ struct kparam_array
 #define core_param_cb(name, ops, arg, perm)		\
 	__level_param_cb(name, ops, arg, perm, 1)
 
+#define kernel_param_cb(name, ops, arg, perm) \
+	__module_param_call("", name, ops, arg, perm, -1, 0)
+
 /**
  * postcore_param_cb - general callback for a module/cmdline parameter
  *                     to be evaluated before postcore initcall level


Or should we change the 'core_param_cb' to what 'kernel_param_cb' is
doing.

Thanks,
Feng