Re: [PATCH, RFC] panic-note: Annotation from user space for panics

Américo Wang
2009-11-18T09:01:22Z

On Wed, Nov 18, 2009 at 8:53 AM, David VomLehn <dvomlehn@cisco.com> wrote:
> On Tue, Nov 17, 2009 at 04:28:22PM -0800, Eric W. Biederman wrote:
>> David VomLehn <dvomlehn@cisco.com> writes:
>>
>> > On Tue, Nov 17, 2009 at 10:45:43AM -0500, Eric W. Biederman wrote:
>> > ...
>> >> Why not use the kdump hook?  If you handle a kernel panic that way
>> >> you get enhanced reliability and full user space support.  All in a hook
>> >> that is already present and already works.
> ...
>> > 1. In what ways would this be better than, say, a panic_notifier?
>>
>> A couple of ways.
>>
>> - You are doing the work in a known good kernel instead of the kernel
>>   that just paniced for some unknown reason.
>> - All of the control logic is in user space (not the kernel) so you can
>>   potentially do something as simple as "date >> logfile" to get the
>>   date.
>
> I think I see better what you're suggesting--passing the info to a kdump
> kernel and having it do whatever it wants. I don't think I want to do this,
> but I haven't used any of the kexec() stuff, so I may be missing the point.
> Some more context:
>
> My application is an embedded one, and one of the big things I need to do
> after a failure is to bring up a fully functional kernel ASAP. Once I have
> that kernel, I process all of the crash data in user space concurrently with
> running my main application. Because I'm embedded, I'm very limited in how
> much crash data I can save over a reboot, how much I can store, and how
> much I can send to a central collection point. This is good, since it doesn't
> take up a lot of resources, but core dumps are out of the question.

I think the problem of kdump is that it uses much memory to hold
the core, i.e. /proc/vmcore, and no way to free it unless using another
reboot. This is why Fedora only does some data-collection in the second
reboot after crash, and then reboots.

I got an idea many days ago, that is providing a way to "delete" /proc/vmcore
in the second reboot, so that we can have enough memory to continue without
another reboot. I am not sure if Eric likes this? Eric?

>
> As I understand kdump, I would also need to have a second kernel in memory
> to do the kdump work. It wouldn't need to be as big is the kernel that
> failed, but it would still require a significant amount of memory. On an
> embedded system, the idle memory may be a luxury we can't afford.

You can use only one kernel, as long as it is relocatable.

>
> I think this makes a kdump-based solution difficult, but if it can meet
> my requirements, I'd much rather use it (I've been following kdump since
> it's inception quite a few years ago, but it hasn't seemed a good match
> for embedded Linux). Does this still sound like a good match?

What do you think about my idea above? If we had that, would kdump
meet your requirements?

>
>> > 2. Where would you suggest tying in? (Particularly since not all architectures
>> >    currently support kdump)
>>
>> No changes are needed kernel side.  You just need an appropriate kernel and
>> initrd for your purpose.
>
> I think I must still be missing something. I have dynamic data that I want
> to preserve as I reboot from a failed kernel to a fresh new kernel. By
> the time the fresh kernel starts init, the dynamic data (IP addresses, MAC
> addresses) has been re-written with new values. This is why I'm trying to
> preserve, but I may be running without disk or flash. This patch doesn't
> preserve the data, but it gets it into the kernel so that it can be
> preserved. At present, I'm preserving the data in a panic_notifier function,
> but I am not wedded to that. At present, the data will be copied to a
> section of memory retained across boots, but I know others will want to
> write to flash.
>

I believe you can get everything from /proc/vmcore, if you use kexec,
after crash, with some tools like 'crash'.

>> All of the interesting architectures support kexec, and if an
>> architecture doesn't it isn't hard to add.  The architecture specific
>> part is very simple.  A pain to debug initially but very simple.
>
> I use MIPS processors, and it looks like it is supported. So long as it's
> stable, I'm happy to use it.

MIPS seems to have some kexec() support, but after looking at
arch/mips/kernel/machine_kexec.c, maybe the support is still
broken? But anyway, you are welcome to work on it. :)