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Date:   Mon, 14 Feb 2022 17:00:37 +0800
From:   Baoquan He <bhe@redhat.com>
To:     "Guilherme G. Piccoli" <gpiccoli@igalia.com>
Cc:     linux-kernel@vger.kernel.org, pmladek@suse.com,
        akpm@linux-foundation.org, anton@enomsg.org, ccross@android.com,
        dyoung@redhat.com, feng.tang@intel.com, john.ogness@linutronix.de,
        keescook@chromium.org, kernel@gpiccoli.net,
        kexec@lists.infradead.org, rostedt@goodmis.org,
        senozhatsky@chromium.org, tony.luck@intel.com, vgoyal@redhat.com
Subject: Re: [PATCH V5] panic: Move panic_print before kmsg dumpers
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References: <20220211215539.822466-1-gpiccoli@igalia.com>
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On 02/11/22 at 06:55pm, Guilherme G. Piccoli wrote:
> The panic_print setting allows users to collect more information in a
> panic event, like memory stats, tasks, CPUs backtraces, etc.
> This is an interesting debug mechanism, but currently the print event
> happens *after* kmsg_dump(), meaning that pstore, for example, cannot
> collect a dmesg with the panic_print extra information.
> 
> This patch changes that in 2 ways:
                               ~~~ steps?

Otherwise, the whole looks straightforward, clear, thanks.

Acked-by: Baoquan He <bhe@redhat.com>

> 
> (a) The panic_print setting allows to replay the existing kernel log
> buffer to the console (bit 5), besides the extra information dump.
> This functionality makes sense only at the end of the panic() function.
> So, we hereby allow to distinguish the two situations by a new boolean
> parameter in the function panic_print_sys_info().
> 
> (b) With the above change, we can safely call panic_print_sys_info()
> before kmsg_dump(), allowing to dump the extra information when using
> pstore or other kmsg dumpers.
> 
> The additional messages from panic_print could overwrite the oldest
> messages when the buffer is full. The only reasonable solution is to
> use a large enough log buffer, hence we added an advice into the kernel
> parameters documentation about that.
> 
> Cc: Baoquan He <bhe@redhat.com>
> Cc: Feng Tang <feng.tang@intel.com>
> Cc: Petr Mladek <pmladek@suse.com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@igalia.com>
> ---
> 
> 
> V5:
> * Rebased against next-20220211.
> * Removed code dealing with kdump, based on Baoquan concerns.
>   This was possible after asking Stephen to remove a patch from
>   linux-next[0] to address Baoquan sugestions, so this version
>   is more simple and doesn't ever panic_print before kdump, unless
>   "crash_kexec_post_notifiers" is passed in the kernel cmdline.
> 
> [0] https://lore.kernel.org/lkml/c10fc4fc-58c9-0b3f-5f1e-6f44b0c190d2@igalia.com/
> 
> V4: https://lore.kernel.org/lkml/20220124203101.216051-1-gpiccoli@igalia.com/
> 
> 
>  Documentation/admin-guide/kernel-parameters.txt |  4 ++++
>  kernel/panic.c                                  | 13 +++++++++----
>  2 files changed, 13 insertions(+), 4 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 3c2b3e24e8f5..2cf7078eaa95 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -3766,6 +3766,10 @@
>  			bit 4: print ftrace buffer
>  			bit 5: print all printk messages in buffer
>  			bit 6: print all CPUs backtrace (if available in the arch)
> +			*Be aware* that this option may print a _lot_ of lines,
> +			so there are risks of losing older messages in the log.
> +			Use this option carefully, maybe worth to setup a
> +			bigger log buffer with "log_buf_len" along with this.
>  
>  	panic_on_taint=	Bitmask for conditionally calling panic() in add_taint()
>  			Format: <hex>[,nousertaint]
> diff --git a/kernel/panic.c b/kernel/panic.c
> index 3c3fb36d8d41..eb4dfb932c85 100644
> --- a/kernel/panic.c
> +++ b/kernel/panic.c
> @@ -148,10 +148,13 @@ void nmi_panic(struct pt_regs *regs, const char *msg)
>  }
>  EXPORT_SYMBOL(nmi_panic);
>  
> -static void panic_print_sys_info(void)
> +static void panic_print_sys_info(bool console_flush)
>  {
> -	if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
> -		console_flush_on_panic(CONSOLE_REPLAY_ALL);
> +	if (console_flush) {
> +		if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
> +			console_flush_on_panic(CONSOLE_REPLAY_ALL);
> +		return;
> +	}
>  
>  	if (panic_print & PANIC_PRINT_ALL_CPU_BT)
>  		trigger_all_cpu_backtrace();
> @@ -286,6 +289,8 @@ void panic(const char *fmt, ...)
>  	 */
>  	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
>  
> +	panic_print_sys_info(false);
> +
>  	kmsg_dump(KMSG_DUMP_PANIC);
>  
>  	/*
> @@ -316,7 +321,7 @@ void panic(const char *fmt, ...)
>  	debug_locks_off();
>  	console_flush_on_panic(CONSOLE_FLUSH_PENDING);
>  
> -	panic_print_sys_info();
> +	panic_print_sys_info(true);
>  
>  	if (!panic_blink)
>  		panic_blink = no_blink;
> -- 
> 2.35.0
>