Re: [PATCH v3 4/8] xfs: flush eof folio before insert range size update

[Brian Foster <bfoster@redhat.com>]

On Mon, Mar 09, 2026 at 10:32:13AM -0700, Darrick J. Wong wrote:
> On Mon, Mar 09, 2026 at 09:45:02AM -0400, Brian Foster wrote:
> > The flush in xfs_buffered_write_iomap_begin() for zero range over a
> > data fork hole fronted by COW fork prealloc is primarily designed to
> > provide correct zeroing behavior in particular pagecache conditions.
> > As it turns out, this also partially masks some odd behavior in
> > insert range (via zero range via setattr).
> > 
> > Insert range bumps i_size the length of the new range, flushes,
> > unmaps pagecache and cancels COW prealloc, and then right shifts
> > extents from the end of the file back to the target offset of the
> > insert. Since the i_size update occurs before the pagecache flush,
> > this creates a transient situation where writeback around EOF can
> > behave differently.
> > 
> > This appears to be corner case situation, but if happens to be
> > fronted by COW fork speculative preallocation and a large, dirty
> > folio that contains at least one full COW block beyond EOF, the
> 
> How do we get a large dirty folio with at least one full cow block
> beyond i_size?  If we did a pagecache write to the file, then at least
> the incore isize should have been boosted out far enough that the block
> will now be inside EOF, right?
> 

It's been quite some time since I first reproduced and diagnosed this so
I'm going from memory, but IIRC it was some odd case like a failure to
split a large folio fully within EOF on a truncate down across it due to
being dirty. I originally thought the large folio thing was actually the
issue, but I don't recall seeing anything that prevents it in this
particular situation. So my understanding from that is that it's more
unexpected in that we wouldn't create a large folio directly in this
situation, but it's still technically possible through oddball fsx
sequences.

From there, we can run into the insert situation described above where
the i_size update -> flush -> extent shift behavior creates a transient
situation where a post-eof block is temporarily within eof.

Brian

> --D
> 
> > writeback after i_size is bumped may remap that COW fork block into
> > the data fork within EOF. The block is zeroed and then shifted back
> > out to post-eof, but this is unexpected in that it leads to a
> > written post-eof data fork block. This can cause a zero range
> > warning on a subsequent size extension, because we should never find
> > blocks that require physical zeroing beyond i_size.
> > 
> > To avoid this quirk, flush the EOF folio before the i_size update
> > during insert range. The entire range will be flushed, unmapped and
> > invalidated anyways, so this should be relatively unnoticeable.
> > 
> > Signed-off-by: Brian Foster <bfoster@redhat.com>
> > Reviewed-by: Christoph Hellwig <hch@lst.de>
> > ---
> >  fs/xfs/xfs_file.c | 17 +++++++++++++++++
> >  1 file changed, 17 insertions(+)
> > 
> > diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
> > index 6246f34df9fd..48d812b99282 100644
> > --- a/fs/xfs/xfs_file.c
> > +++ b/fs/xfs/xfs_file.c
> > @@ -1263,6 +1263,23 @@ xfs_falloc_insert_range(
> >  	if (offset >= isize)
> >  		return -EINVAL;
> >  
> > +	/*
> > +	 * Let writeback clean up EOF folio state before we bump i_size. The
> > +	 * insert flushes before it starts shifting and under certain
> > +	 * circumstances we can write back blocks that should technically be
> > +	 * considered post-eof (and thus should not be submitted for writeback).
> > +	 *
> > +	 * For example, a large, dirty folio that spans EOF and is backed by
> > +	 * post-eof COW fork preallocation can cause block remap into the data
> > +	 * fork. This shifts back out beyond EOF, but creates an expectedly
> > +	 * written post-eof block. The insert is going to flush, unmap and
> > +	 * cancel prealloc across this whole range, so flush EOF now before we
> > +	 * bump i_size to provide consistent behavior.
> > +	 */
> > +	error = filemap_write_and_wait_range(inode->i_mapping, isize, isize);
> > +	if (error)
> > +		return error;
> > +
> >  	error = xfs_falloc_setsize(file, isize + len);
> >  	if (error)
> >  		return error;
> > -- 
> > 2.52.0
> > 
> > 
>