xfs resize: primary superblock is not updated immediately

[Dave Chinner]

On Mon, Feb 22, 2016 at 09:08:06PM +0200, Alex Lyakas wrote:
> Greetings XFS developers,
> 
> I am seeing the following issue with XFS on kernel 3.18.19.
> 
> When resizing, XFS adds new AGs and eventually updates the primary
> superblock with the new “sb_agcount” value. However, it happens few
> seconds after the resize operation completes back to user-space. As
> a result, if a block-level snapshot is taken off the underlying
> block device, while “sb_agcount” still has the old value, then
> subsequent XFS mount crashes with stack like[1].

The primary superblock change is logged, so it doesn't need to be
written back immediately. That means it is in the journal...

> Some debugging shows that _xfs_buf_find is called with agno that has
> been added during the resize, but appropriate "pag" has not been
> created for this agno during mount.

The new per-ag structures are created during growfs, after the
growfs transaction has committed. if you are mounting a snapshot
that has the wrong agcount in it, then lots of things will go wrong
if there is metadata that already uses the expanded space.

> I have found the patch by Christoph Hellwig:
> http://oss.sgi.com/archives/xfs/2015-01/msg00391.html
> which sets the resize transaction to be synchronous, and applied it,
> but it still doesn’t help.
> 
> Right after the resize completes, I am issuing:
> xfs_db -r -c "sb 0" -c "p"   <device>
> and for a few seconds still get the old value of “sb_agcount”.
> 
> Can anybody advise what am I missing? What needs to be done so that
> the primary superblock will get the new value of “sb_agount”
> promptly?

Are you freezing the filesystem before taking a block level
snapshot?

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com