linux-kernel - Re: [PATCH v3 5/5] panic: add note that panic

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Message-ID: <aJvk2ti-D3t3EUsX@U-2FWC9VHC-2323.local>
Date: Wed, 13 Aug 2025 09:05:30 +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 v3 5/5] panic: add note that panic_print sysctl interface
 is deprecated

On Tue, Aug 12, 2025 at 01:52:58PM +0200, Petr Mladek wrote:
> On Thu 2025-07-03 10:10:04, Feng Tang wrote:
> > Add a dedicated core parameter 'panic_console_replay' for controlling
> > console replay, and add note that 'panic_print' sysctl interface  will
> > be obsoleted by 'panic_sys_info' and 'panic_console_replay'.  When it
> > happens, the SYS_INFO_PANIC_CONSOLE_REPLAY can be removed as well.
> > 
> > --- a/kernel/panic.c
> > +++ b/kernel/panic.c
> > @@ -77,6 +78,13 @@ 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)
> > +{
> > +	pr_info_once("Kernel: 'panic_print' sysctl interface will be obsoleted by both 'panic_sys_info' and 'panic_console_replay'\n");
> > +	return proc_doulongvec_minmax(table, write, buffer, lenp, ppos);
> > +}
> 
> This warning is printed "only" when the value is accessed via the
> procfs. It would be great to print it also when it is set
> via the command line parameter.

Yes, this is indeed a remaining issue to be solved, as mentioned in
the cover letter.

> 
> It would require replacing
> 
> core_param(panic_print, panic_print, ulong, 0644);
> 
> with
> 
> core_param_cb(panic_print, &panic_print_ops, &panic_print, 0644);

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 adding a new 'kernel_param_cb' can work without
the "panic" prefix, 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

Thanks,
Feng