[PATCH V2] panic: Move panic_print before kmsg dumpers

[Guilherme G. Piccoli]

The panic_print setting allows users to collect more information in a
panic event, like memory stats, tasks, CPUs backtraces, etc.
This is a pretty 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 information.

This patch changes that in 2 ways:

(a) First, after a good discussion with Petr in the mailing-list[0],
he noticed that one specific setting of panic_print (the console replay,
bit 5) should not be executed before console proper flushing; hence we
hereby split the panic_print_sys_info() function in upper and lower
portions: if the parameter "after_kmsg_dumpers" is passed, only bit 5
(the console replay thing) is evaluated and the function returns - this
is the lower portion. Otherwise all other bits are checked and the
function prints the user required information; this is the upper/earlier
mode.

(b) With the above change, we can safely insert a panic_print_sys_info()
call up some lines, in order kmsg_dump() accounts this new information
and exposes it through pstore or other kmsg dumpers. Notice that this
new earlier call doesn't set "after_kmsg_dumpers" so we print the
information set by user in panic_print, except the console replay that,
if set, will be executed after the console flushing.

In the first version of this patch we discussed the risks in the
mailing-list[0], and seems this approach is the least terrible,
despite still having risks of polluting the log with the bulk of
information that panic_print may bring, losing older messages.
In order to attenuate that, we've added a warning in the
kernel-parameters.txt so users enabling this mechanism consider
to increase the log buffer size via "log_buf_len" as well.

[0] https://lore.kernel.org/lkml/20211230161828.121858-1-gpiccoli@igalia.com

Cc: Feng Tang <feng.tang at intel.com>
Cc: Petr Mladek <pmladek at suse.com>
Signed-off-by: Guilherme G. Piccoli <gpiccoli at igalia.com>
---


V2:
- Rebased to linux-next (20220106), since there are patches
on linux-next touching this portion of code.

- Better document the risks of panic_print and suggest users
to increase log buffer size with log_buf_len when using it.

- "Split" panic_print_sys_info() in order to execute the info
collection in the upper part (allowing kmsg dumpers to save
that information) and the console replay in the lower part,
after the console flush.

Petr, thanks a lot for your review and suggestions!
This is my idea as I've mentioned in the V1 thread, but if you
think the parameter is better, i.e, decouple the console replay
from panic_print, let me know and I can rework - your choice.
I just wanted to avoid adding  yet another kernel parameter,
since it's not strictly needed heh

Cheers!


 Documentation/admin-guide/kernel-parameters.txt |  4 ++++
 kernel/panic.c                                  | 15 ++++++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a069d8fe2fee..0f5cbe141bfd 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -3727,6 +3727,10 @@
 			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 41ecf9ab824a..fc055a91c103 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 after_kmsg_dumpers)
 {
-	if (panic_print & PANIC_PRINT_ALL_PRINTK_MSG)
-		console_flush_on_panic(CONSOLE_REPLAY_ALL);
+	if (after_kmsg_dumpers) {
+		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();
@@ -249,7 +252,7 @@ void panic(const char *fmt, ...)
 	 * show some extra information on kernel log if it was set...
 	 */
 	if (kexec_crash_loaded())
-		panic_print_sys_info();
+		panic_print_sys_info(false);
 
 	/*
 	 * If we have crashed and we have a crash kernel loaded let it handle
@@ -283,6 +286,8 @@ void panic(const char *fmt, ...)
 	 */
 	atomic_notifier_call_chain(&panic_notifier_list, 0, buf);
 
+	panic_print_sys_info(false);
+
 	kmsg_dump(KMSG_DUMP_PANIC);
 
 	/*
@@ -313,7 +318,7 @@ 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;
-- 
2.34.1