Re: Announce: dumpfs v0.01 - common RAS output API

[ebiederm@xmission.com (Eric W. Biederman)]

Andrew Morton <akpm@osdl.org> writes:

> Keith Owens <kaos@sgi.com> wrote:
> >
> >  * How do we get a clean API to do polling mode I/O to disk?
> 
> We hope to not have to.  The current plan is to use kexec: at boot time, do
> a kexec preload of a small (16MB) kernel image.  When the main kernel
> crashes or panics, jump to the kexec kernel.  The kexec kernel will hold a
> new device driver for /dev/hmem through which applications running under
> the kexec'ed kernel can access the crashed kernel's memory.

Hmm.  I think this will require one of the kernels to run at a
non-default address in physical memory.

> Write the contents of /dev/hmem to stable storage using whatever device
> drivers are in the kexeced kernel, then reboot into a real kernel
> again.

And at this point I don't quite see why you would need /dev/hmem,
as opposed to just using /dev/mem.

Or will the crashing kernel save and compress the core dump to
somewhere in ram and the dump kernel read it out from there? 

> That's all pretty simple to do, and the quality of the platform's crash
> dump feature will depend only upon the quality of the platform's kexec
> support.

Which will largely depend on the quality of it's device drivers...
 
> People have bits and pieces of this already - I'd hope to see candidate
> patches within a few weeks.  The main participants are rddunlap, suparna
> and mbligh.

I'm sorry I missed you then.  Unfortunately this is my busiest season at work
so I wasn't able to make it to OLS this year :(

Does anyone have a proof of concept implementation?  I have been able to find
a little bit of time for this kind of thing lately and have just done
the x86-64 port.  (You can all give me a hard time about taking a year
to get back to it :)  I am in the process of breaking everything up
into their individual change patches and doing a code review so I feel
comfortable with sending the code to Andrew.  So this would be a very
good time for me to look at any code for reporting a crash dump with
a kernel started with kexec.

Eric