Re: [BUG] xfs/109 crashed 2k block size reflink enabled XFS

[Christoph Hellwig <hch@infradead.org>]

On Mon, Dec 05, 2016 at 06:39:06AM -0800, Christoph Hellwig wrote:
> On Mon, Dec 05, 2016 at 05:21:12PM +0800, Eryu Guan wrote:
> > Hi,
> > 
> > I hit an xfs/109 crash today while testing reflink XFS with 2k block
> > size on x86_64 hosts (both baremetal and kvm guest).
> > 
> > It can be reproduced by running xfs/109 many times, I tried 50-times
> > loop twice and it crashed at the 21st and 46th runs. And I can reproduce
> > it with both linus tree (4.9-rc4) and linux-xfs tree for-next branch
> > (updated on 2016-11-30). I haven't been able to reproduce it with 4k
> > block size XFS.
> 
> Haven't been able to reproduce it yet unfortunately.  But from looking
> at the out of range block this looks like it could be NULLFSBLOCK
> converted to a daddr.
> 
> I assume you are running without CONFIG_XFS_DEBUG or CONFIG_XFS_WARN
> enabled?
> 
> Below would catch this issue in a non-debug build.  Still trying to
> reproduce in the meantime..

Ok, finally managed to reproduce it and hit my BUG_ON below after
hitting the the "trying another AG" printk (only once so far).

Seems like the retry case for COW file systems doesn't work reliably.
That being said given that this tests doesn't even exercise the COW
functionality we shouldn't normally even hit this case, should we?