Re: How to get the crash dump if system hangs?

[Muni Sekhar <munisekharrms@gmail.com>]

On Fri, Oct 4, 2019 at 3:06 AM Kees Cook <keescook@chromium.org> wrote:
>
> On Thu, Oct 03, 2019 at 10:18:48PM +0530, Muni Sekhar wrote:
> > Thanks a lot for letting me know about pstore, will try this option.
> > It will be helpful if you can share some pointers on 'how to enable
> > software ECC'?
>
> When I boot with pstore, I use a bunch of command line arguments to test
> all its feature:
>
> ramoops.mem_size=1048576
> ramoops.ecc=1
> ramoops.mem_address=0x440000000
> ramoops.console_size=16384
> ramoops.ftrace_size=16384
> ramoops.pmsg_size=16384
> ramoops.record_size=32768
>
> but I'm using pmem driver to reserve the 1MB of memory at 0x440000000.
>
> To do a RAM reservation on a regular system, you'll need to do something
> like boot with:
>
> memmap=1M!1023M
>
> which says, reserve 1MB of memory at the 1023M offset. So this depends
> on how much physical memory you have, etc, but you'll be able to see the
> reservation after booting in /proc/iomem. e.g. for me, before:
>
> ...
> 00100000-bffd9fff : System RAM
> ...
>
> with memmap:
>
> ...
> 00100000-3fefffff : System RAM
> 3ff00000-3fffffff : Persistent Memory (legacy)
> 40000000-bffd9fff : System RAM
> ...
>
> So in that example, the address you'd want is 0x3ff00000
>
> memmap=1M!1023M
> ramoops.mem_size=1048576
> ramoops.ecc=1
> ramoops.mem_address=0x3ff00000
> ramoops.console_size=16384
> ramoops.ftrace_size=16384
> ramoops.pmsg_size=16384
> ramoops.record_size=32768
>
> In dmesg you should see:
>
> [    0.868818] pstore: Registered ramoops as persistent store backend
> [    0.869713] ramoops: using 0x100000@0x3ff00000, ecc: 16
>
> And if that address lines up with the "Persistent Memory (legacy)" line
> in /proc/iomem you should be good to go.
>
> Just mount /sys/fs/pstore and see if the console dump updates between
> warm boots, then try some cold boots, see if the ECC works, etc.
>
> Good luck!
>
> --
> Kees Cook

Thanks for the answers.

My kernel is configured with following .config options:

CONFIG_EFI_VARS_PSTORE=y
# CONFIG_EFI_VARS_PSTORE_DEFAULT_DISABLE is not set
CONFIG_PSTORE=y
CONFIG_PSTORE_CONSOLE=y
CONFIG_PSTORE_PMSG=y
# CONFIG_PSTORE_FTRACE is not set
CONFIG_PSTORE_RAM=y

Before RAM reservation, I see the following in /proc/iomem :
# cat iomem | grep "System RAM"

00001000-0009d7ff : System RAM
00100000-1fffffff : System RAM
20100000-b937dfff : System RAM
b9ba6000-b9ba6fff : System RAM
b9be9000-b9d5dfff : System RAM
b9ffa000-b9ffffff : System RAM
100000000-13fffffff : System RAM

Later I booted with “memmap=1M!1023M ramoops.mem_size=1048576
ramoops.ecc=1 ramoops.mem_address=0x3ff00000
ramoops.console_size=16384 ramoops.ftrace_size=16384
ramoops.pmsg_size=16384 ramoops.record_size=32768 ramoops.mem_type=1
ramoops.dump_oops=1”

After reboot, In dmesg I see the following lines:

[    0.373084] pstore: Registered ramoops as persistent store backend
[    0.373266] ramoops: attached 0x100000@0x3ff00000, ecc: 16/0

# cat /proc/iomem | grep "System RAM"
00001000-0009d7ff : System RAM
00100000-1fffffff : System RAM
20100000-3fefffff : System RAM
3ff00000-3fffffff : Persistent RAM
40000000-b937dfff : System RAM
b9ba6000-b9ba6fff : System RAM
b9be9000-b9d5dfff : System RAM
b9ffa000-b9ffffff : System RAM
100000000-13fffffff : System RAM

I noticed Persistent RAM, not Persistent Memory (legacy). What is the
difference between these two?

I could not find any file in /sys/fs/pstore after warm boot. Even
tried to trigger the crash by running “echo c > /proc/sysrq-trigger”
and then rebooted  the system manually. After system boots up, I could
not find dmesg-ramoops-N file in /sys/fs/pstore, even I could not find
any file in /sys/fs/pstore directory.

Am I missing anything?

-- 
Thanks,
Sekhar