generic/04[89] fail on XFS due to change in writeback code [4.2-rc1 regression]

[Tejun Heo]

On Thu, Aug 13, 2015 at 11:34:42AM -0400, Tejun Heo wrote:
> On Thu, Aug 13, 2015 at 10:44:35AM +1000, Dave Chinner wrote:
> > It might be a couple of days before I really get a chance to dig
> > into this, so it might be best if Tejun can look into it first.
> 
> Yeap, have been looking into it since yesterday.  I have some
> suspicions.  I'll write once I know more.

So, here are what I've found out till now.

* I can't reproduce it for some reason.

* There's a bug in b_dirty_time handling.  sync_inodes_sb() should
  schedule writebacks regardless of b_dirty_time but it currently
  isn't.  I'm working on a patch to fix it.

* But I can't see how the above bug would lead to failure of size
  sync.  One possibility is that wb_has_dirty_io() and/or
  bdi_has_dirty_io() is getting out of sync for some reason.  I'll
  write up a debug patch for this.

Thanks.

-- 
tejun