Re: [PATCH 6/7] xfs: don't ENOSPC on writeback when punching holes

[Christoph Hellwig <hch@infradead.org>]

On Tue, Nov 20, 2018 at 08:50:42PM +1100, Dave Chinner wrote:
> > > because the writeback error of ENOSPC was being reported. We're
> > > going to free that space, so we don't care if there was a ENOSPC
> > > writeback error. So ignore ENOSPC errors and punch anyway.
> > 
> > How do even get -ENOSPC back from writeback?  It seems like we have
> > a much worse root cause lingering here.
> 
> It hammered ENOSPC pretty hard - I think it had consumed the entire
> reserve pool and that can causes allocation transaction reservations
> in xfs_iomap_write_allocate() to return ENOSPC even if we've got a
> reservation for the data extents being allocated.

Well, that means we do have a problem somewhere in our accounting,
as writeback should not run into ENOSPC.

> This doesn't happen very often in the real world, but if it does we
> need operations like punch to work to be able to free space and
> get us out of that hole...

I'm ok(-ish) with the patch, but I wish we could also sort out the
root cause..