exception-trace-always-printk-segfault-info.patch

From: Coly Li <bosong.ly@taobao.com>
Subject: [PATCH] exception-trace: always show segfault info in dmesg if /proc/sys/debug/exception-trace set to 2
Date: Tue May  8 17:32:27 CST 2012
Patch-mainline: in-house usage

JVM (Java virtual machine) registers its own signal handlers, in case of SIGSEGV which a jvm process should be
received, kernel may not always display segfault info in dmesg if unhandled_signal() or printk_ratelimit() return
fauls.

sysctl file /proc/sys/debug/exception-trace controls segfault dmesg behavior, current configure in RHEL6 kernel is,
*) 0, don't display segfault info in dmesg
*) 1, may display segfault info in dmesg, if pass unhandled_signal() and printk_ratelimit() checking.
This patch add another configuration,
*) 2, always display segfault info in dmesg, skip unhandled_signal() and printk_ratelimit() checking.

This is a special requirement from JVM development/maintenance team. exception-trace is default to 1, if users
want always display segfault info, please configure /proc/sys/debug/exception-trace to 2.

Signed-off-by: Coly Li <bosong.ly@taobao.com>
---
Index: linux-2.6.32-279.2.1.el5/arch/x86/mm/fault.c
===================================================================
--- linux-2.6.32-279.2.1.el5.orig/arch/x86/mm/fault.c
+++ linux-2.6.32-279.2.1.el5/arch/x86/mm/fault.c
@@ -706,12 +706,21 @@ static inline void
 show_signal_msg(struct pt_regs *regs, unsigned long error_code,
 		unsigned long address, struct task_struct *tsk)
 {
+	/*
+	 * if show_unhandled_signals == 2, always printk segfault
+	 * information in dmesg, this is required by java virtual
+	 * machine developers.
+	 */
+	if (unlikely(show_unhandled_signals == 2))
+		goto show_msg;
+
 	if (!unhandled_signal(tsk, SIGSEGV))
 		return;
 
 	if (!printk_ratelimit())
 		return;
 
+show_msg:
 	printk("%s%s[%d]: segfault at %lx ip %p sp %p error %lx",
 		task_pid_nr(tsk) > 1 ? KERN_INFO : KERN_EMERG,
 		tsk->comm, task_pid_nr(tsk), address,