Re: [Fastboot] [PATCH] Kdump(x86): add note type NT_KDUMPINFO tokernelcore dumps

[Vivek Goyal <vgoyal@in.ibm.com>]

On Fri, Sep 23, 2005 at 01:12:23AM -0600, Eric W. Biederman wrote:
> Dave Anderson <anderson@redhat.com> writes:
> 
> > So does elf_core_dump() as well, but to gdb it's useless AFAICT...
> 
> We can always post_process things when generating a core dump
> if we have enough information.
> 
> > Hey -- I wasn't even aware of the "crashing_cpu" variable.  
> > That would work just fine.
> >
> > Still a "panic_task", and perhaps even a "crash_page_size" variable
> > would be nice as well.   No additional notes required...
> 
> To avoid defining an ABI that we need to maintain there is some
> benefit in simply using static variables.  But the form of the
> information really isn't the concern.
> 
> Where we capture the information and how reliable is that capture
> is the concern.
> 
> To capture page size the easiest and most reliable way I can see
> to do is to modify vmlinux.lds.S to contain something like:
> >	 _page_shift = PAGE_SHIFT;
> Giving you an absolute symbol _page_shift in vmlinux that
> contains the value you need, without overhead in the running
> kernel.

That's a cool idea. I just tested it. 

[vivek@vivegoya linux]$ readelf -s vmlinux | grep __page_shift
 28865: 0000000c     0 NOTYPE  GLOBAL DEFAULT  ABS __page_shift

> 
> crashing_cpu makes sense to capture in some form, we definitely
> need to compute something that will allow us to write to
> a per cpu area on an SMP system.
> 
> The big concern at this point is that the code has not undergone
> a serious stability audit.  So it is the expectation that there
> is still code we can remove and modify to increase the likely hood
> of getting a crash dump.
> 
> Currently we know that stack overflows sometimes happen and that
> they are a source of kernel crashes.  It would be good if we could
> take a crash dump despite them.  To do that requires code more
> robust than we have today.  Quite likely it means that we will
> not be able to reliably capture the task_struct of the crashing cpu.
>

Agreed.