Heads-up: Possible regression between 10.0-RELEASE and 10.1-BETA1 with '/ on ZFS' setup

[Glen Barber]

On Sat, Oct 04, 2014 at 05:15:30PM -0400, Glen Barber wrote:
> On Sat, Oct 04, 2014 at 08:46:00PM +0300, Konstantin Belousov wrote:
> > On Sat, Oct 04, 2014 at 10:03:48AM -0700, Peter Wemm wrote:
> > > > If we cannot increase KSTACK_PAGES by default, do we have any
> > > > alternative solution outside of suggesting to avoid using ZFS on i386
> > > > with more than one disk?
> > > 
> > > When zfs creates its kthreads it can specify how much stack it needs.  For 
> > > i386 it could ask for more for the zfs threads.  Its not a good option but its 
> > > better than more stack for everything when it's already easy to run out 
> > > without zfs.
> > 
> > This one probably happens in the init thread, not some of the zfs hord.
> > Glen did not show the backtrace from ddb yet (I hope that ddb did not
> > regressed and can step over double-fault boundary).
> > 
> > We could specifically increment the init thread stack size as well, but
> > I have no idea if normal VFS calls into ZFS are affected and cause overflow
> > for the normal threads after the multitasking is fired.
> 
> As soon as I get the kernel built with debugging support, I'll be able
> to get the backtrace.  This, however, is proving to be non-trivial.
> 

I was finally able to get a VM configured in a way that would allow
getting a ddb backtrace after panic.

Screenshots of the panic are available here:

https://people.freebsd.org/~gjb/i386-zfs/zfs-i386-1.png
https://people.freebsd.org/~gjb/i386-zfs/zfs-i386-2.png
https://people.freebsd.org/~gjb/i386-zfs/zfs-i386-3.png
https://people.freebsd.org/~gjb/i386-zfs/zfs-i386-4.png

I'll leave the VM in its current state in case more information is
needed.

I also want to point out that 10.0-RELEASE also crashes upon mounting
the ZFS filesystems when the machine boots after an unclean shutdown, so
although the behavior of "when" the machine crashes has changed, I am
not certain this is a regression since 10.0-RELEASE.

Glen

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