Re: [PATCH 3/6] xfs: flush eof folio before insert range size update

["Darrick J. Wong" <djwong@kernel.org>]

On Thu, Oct 16, 2025 at 03:03:00PM -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.

Why not flush the file from @offset to EOF, flush the COW
preallocations, extend i_size, and only then start shifting extents?
That would seem a lot more straightforward to me.

--D

> 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
> 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>
> ---
>  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 5b9864c8582e..cc3a9674ad40 100644
> --- a/fs/xfs/xfs_file.c
> +++ b/fs/xfs/xfs_file.c
> @@ -1226,6 +1226,23 @@ 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;
> -- 
> 2.51.0
> 
>