Re: [gentoo-user] kernel panick in 4.2.1 from gentoo-sources

[Rich Freeman]

On Wed, Oct 7, 2015 at 4:13 PM,  <covici@××××××××××.com> wrote:
> Hi.  I am getting some kind of kernel panick in 4.2.1 -- it boots up OK,
> ...
> how
> do I get any information about what happened -- serial console or other
> means?  Can I do a console over the network without additional hardware?

That is pretty simple actually.

Set CONFIG_NETCONSOLE=y/m in your kernel config if it is not already set.
add to your kernel command line:
netconsole=6666@192.168.0.10/eth0,6666@192.168.0.5/1c:6f:65:ab:07:b2

(The first set of values is port@src-ip/interface.  The second set of
values is port@dest-ip/MAC.  This is low-level code in the kernel so
it is just sending raw UDP packets - the routine sending them has no
idea what your interface IP is, and it can't use ARP.)

On the destination machine, run "nc -u -l -p 6666"

That will listen for console output and dump it to stdout.  You'll get
everything that goes to dmesg on the remote machine, including
BUG/PANIC/etc output.  It works fine even if the disks stop syncing.

>
> The reason I went with that kernel is because I want to try btrfs and
> they develop fast, so it looked from Google searching that I should be
> on 4.2 or thereabouts.  The btrfs programs I emerged did say 4.2.
>

If you're having btrfs issues on such a recent kernel you should
probably at least run all the backports that are available for it.

There are undoubtedly many btrfs issues in 4.2.1 that have been fixed
in 4.2.3, so you should probably be running this version if you want
to stick with 4.2.

Personally, I've been sticking with 3.18 until 4.1 quiets down.  There
are usually regressions in any new kernel version with btrfs.

> So, I would like to go on two paths at once -- find out about the
> panick, and maybe go to a lower kernel as well, but I was concerned
> about btrfs if I do that.  I have not created the pool yet.

Generally speaking the btrfs on-disk format is stable, so for the most
part you can switch back and forth between versions without issue.  If
you want to go to a really old kernel series like maybe 3.12 there
might be a few optional btrfs features that won't work, but in general
I'd stick with something newer.

So, if you want to be bleeding-edge then stick with the bleeding edge
and run the latest stable.  If you want something longterm I'd stick
with the 1st-2nd most recent longterm.  4.1 is still pretty new, but
I'm close to switching over to it.

You'd need to post the details of the panic to know more - the btrfs
list is probably the best place.  But again I'd confirm the panic on
the latest release in the series you're running so as to not waste
time on issues that may already be fixed.

-- 
Rich