Subject    : kexec/kdump kernel fails to start
Submitter  : Flavio Leitner <fbl@redhat.com>
Date       : Tue, 4 Sep 2012 14:32:15 -0300
Message-ID : <20120904143215.5bbbb2a4@obelix.rh>
References : https://lkml.org/lkml/2012/9/4/276

Hi folks,

I have system that no longer boots kdump kernel. Basically,

# echo c > /proc/sysrq-trigger

to dump a vmcore doesn't work. It just hangs after showing the usual
panic messages. I've bisected the problem and the commit introducing
the issue is the one below.

Any idea?

commit 722bc6b16771ed80871e1fd81c86d3627dda2ac8
Author: WANG Cong <xiyou.wangcong@gmail.com>  2012-03-05 20:05:13
Committer: Ingo Molnar <mingo@elte.hu>  2012-03-06 05:38:26
Parent: 550cf00dbc8ee402bef71628cb71246493dd4500 (Merge tag 'mmc-fixes-for-3.3' of git://git.kernel.org/pub/scm/linux/kernel/git/cjb/mmc)
Child:  a6fca40f1d7f3e232c9de27c1cebbb9f787fbc4f (x86, tlb: Switch cr3 in leave_mm() only when needed)
Branches: master, remotes/origin/master
Follows: v3.3-rc6
Precedes: v3.5-rc1

    x86/mm: Fix the size calculation of mapping tables
    For machines that enable PSE, the first 2/4M memory region still uses
    4K pages, so needs more PTEs in this case, but
    find_early_table_space() doesn't count this.
    
    This patch fixes it.
    
    The bug was found via code review, no misbehavior of the kernel
    was observed.


Machine details:
processor       : 0
vendor_id       : GenuineIntel
cpu family      : 6
model           : 26
model name      : Intel(R) Core(TM) i7 CPU         920  @ 2.67GHz
stepping        : 5
microcode       : 0x11
cpu MHz         : 1596.000
cache size      : 8192 KB
physical id     : 0
siblings        : 8
core id         : 0
cpu cores       : 4
apicid          : 0
initial apicid  : 0
fpu             : yes
fpu_exception   : yes
cpuid level     : 11
wp              : yes
flags           : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc aperfmperf pni dtes64 monitor ds_cpl vmx est tm2 ssse3 cx16 xtpr pdcm sse4_1 sse4_2 popcnt lahf_lm ida dts tpr_shadow vnmi flexpriority ept vpid
bogomips        : 5333.87
clflush size    : 64
cache_alignment : 64
address sizes   : 36 bits physical, 48 bits virtual
power management:

# free
             total       used       free     shared    buffers     cached
Mem:      16161684   11749100    4412584          0      10212   11421096
-/+ buffers/cache:     317792   15843892
Swap:     17406420          0   17406420


dmesg is attached.

thanks,
fbl