Re: md-raid5 with bcache member devices => kernel panic

[Kent Overstreet <kmo@daterainc.com>]

On Fri, Dec 06, 2013 at 12:08:13AM +0100, Matthias Ferdinand wrote:
> On Thu, Dec 05, 2013 at 02:52:34PM -0800, Kent Overstreet wrote:
> > On Thu, Dec 05, 2013 at 10:29:13PM +0100, Matthias Ferdinand wrote:
> > Erk. I thought I was done with these bugs. Nick, do you think you could try and
> > track this down?
> > 
> > Looking at this:
> > http://dl.mfedv.net/md5raid_on_bcache_panic/mdraid5_on_bcache_panic_3.12.2.txt
> > 
> > that's a null pointer deref; if Matthias could get the exact line number it
> I have no idea how to do that - can you help me out on that?
> 
> > Matthias - I'm running bcache on top of a raid6 at home and I've never seen
> I only get panics when I use md-raid on top of bcaches:
> 
>               LVM
>                |
>            md-raid5
>          /     |     \
>   bcache0  bcache1  bcache2
>    |           |       |
>   sdb6       sdc6     sdd6
> 
> (probably an unusual setup; just playing around...)

Yeah, that is unusual. Very odd though, that setup I would definitely expect to
work.

You mentioned it's faster with bcache higher in the stack - do you have any
issues with your preferred setup?

If the bug isn't actually affecting real/preferred use cases then I'll be a lot
less concerned about it - I'm reworking how all this crap works in mainline,
maybe by 3.14 generic_make_request() will be accepting arbitrary size bios and
the code that's probably buggy here will be gone.