Re: [PATCH v3 3/3] xfs: correct the zeroing truncate range

[Dave Chinner <david@fromorbit.com>]

On Tue, May 21, 2024 at 08:00:20PM -0700, Darrick J. Wong wrote:
> On Tue, May 21, 2024 at 12:38:30PM +1000, Dave Chinner wrote:
> > [RFC] iomap: zeroing needs to be pagecache aware
> > 
> > From: Dave Chinner <dchinner@redhat.com>
> > 
> > Unwritten extents can have page cache data over the range being
> > zeroed so we can't just skip them entirely. Fix this by checking for
> > an existing dirty folio over the unwritten range we are zeroing
> > and only performing zeroing if the folio is already dirty.
> > 
> > XXX: how do we detect a iomap containing a cow mapping over a hole
> > in iomap_zero_iter()? The XFS code implies this case also needs to
> > zero the page cache if there is data present, so trigger for page
> > cache lookup only in iomap_zero_iter() needs to handle this case as
> > well.
> 
> Hmm.  If memory serves, we probably need to adapt the
> xfs_buffered/direct_write_iomap_begin functions to return the hole in
> srcmap and the cow mapping in the iomap.  RN I think it just returns the
> hole.

Yes, that is what I was thinking we need to do -
xfs_buffered_write_iomap_begin() doesn't even check for COW mappings
if IOMAP_ZERO is set, so there's a bunch of refactoring work needed
to let iomap know that there is a COW mapping over the hole so it
can do the same page cache lookup stuff that I added for unwritten
extents....

-Dave.

-- 
Dave Chinner
david@fromorbit.com