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

[Suparna Bhattacharya <suparna@in.ibm.com>]

On Tue, Jul 27, 2004 at 07:53:01PM -0600, Eric W. Biederman wrote:
> 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.

This differs a little from your earlier suggestion of requiring
a kernel to run from a non-default address. Martin suggested simply
reserving about 16MB of area in advance, so that just before kexecing
the new kernel with mem=16M, we save the first 16MB away into the
reserved space. /dev/hmem (oldmem ?) is a view into the old kernel's
memory, as opposed to /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

Yes, Hari has a nice POC implementation - it might make sense for him to post
it rightaway for you to take a look. Basically, in addition to hmem (oldmem),
the upcoming kernel exports an ELF core view of the saved register and memory 
state of the previous kernel as /proc/vmcore.prev (remember your suggestion 
of using an ELF core file format for dump ?), so one can use cp or scp to 
save the core dump to disk. He has a quick demo, where he uses gdb (unmodified) 
to open the dump and show a stack trace of the dumping cpu.

Regards
Suparna

> 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

-- 
Suparna Bhattacharya (suparna@in.ibm.com)
Linux Technology Center
IBM Software Lab, India