Re: [PATCH 10/10] xfs: retry COW fork delalloc conversion when no

["Darrick J. Wong" <darrick.wong@oracle.com>]

On Mon, Feb 18, 2019 at 10:09:42AM +0100, Christoph Hellwig wrote:
> On Fri, Feb 15, 2019 at 03:32:25PM -0800, Darrick J. Wong wrote:
> > I /think/ the way that this series (+ Brian's before that) solve the
> > truncate/writeback race is that we now only convert existing delalloc
> > reservations to real extents when we're preparing to do writeback;
> > _writepage_map only cares about the mapping of the offset_fsb that it
> > happens to be looping right now (because the page lock serializes with
> > page cache truncate/punch); and we use the new sequence counters for
> > both the data and cow forks to decide when our cached mapping might be
> > invalid and therefore we need to get a new mapping?
> 
> Well, we already tried to do that before, we just weren't all that good
> at it.  The big difference is that the delalloc conversion now doesn't
> blindly reuse the range looked up a long time before under a different
> ilock critical section, but instead just uses that as a hint and
> only converts the extent that the writeback offset falls into, and
> only does so if it actually still is in delalloc state.

Got it.  I'll pull this in if I don't hear any loud yelling. :)

(FWIW it tested ok over the weekend.)

--D