Re: [PATCH v3 1/8] xfs: fix iomap hole map reporting for zoned zero range

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

On Mon, Mar 09, 2026 at 09:44:59AM -0400, Brian Foster wrote:
> The hole mapping logic for zero range in zoned mode is not quite
> correct. It currently reports a hole whenever one exists in the data
> fork. If the first write to a sparse range has completed and not yet
> written back, the blocks exist in the COW fork as delalloc until
> writeback completes, at which point they are allocated and mapped
> into the data fork. If a zero range occurs on a range that has not
> yet populated the data fork, we will incorrectly report it as a
> hole.
> 
> Note that this currently functions correctly because we are bailed
> out by the pagecache flush in iomap_zero_range(). If a hole or
> unwritten mapping is reported with dirty pagecache, it assumes there
> is pending data, flushes to induce any pending block
> allocations/remaps, and retries the lookup. We want to remove this
> hack from iomap, however, so update iomap_begin() to only report a
> hole for zeroing when one exists in both forks.
> 
> Signed-off-by: Brian Foster <bfoster@redhat.com>
> ---
>  fs/xfs/xfs_iomap.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
> index be86d43044df..8c3469d2c73e 100644
> --- a/fs/xfs/xfs_iomap.c
> +++ b/fs/xfs/xfs_iomap.c
> @@ -1651,14 +1651,6 @@ xfs_zoned_buffered_write_iomap_begin(
>  				&smap))
>  			smap.br_startoff = end_fsb; /* fake hole until EOF */
>  		if (smap.br_startoff > offset_fsb) {
> -			/*
> -			 * We never need to allocate blocks for zeroing a hole.
> -			 */
> -			if (flags & IOMAP_ZERO) {
> -				xfs_hole_to_iomap(ip, iomap, offset_fsb,
> -						smap.br_startoff);
> -				goto out_unlock;
> -			}
>  			end_fsb = min(end_fsb, smap.br_startoff);
>  		} else {
>  			end_fsb = min(end_fsb,
> @@ -1690,6 +1682,16 @@ xfs_zoned_buffered_write_iomap_begin(
>  	count_fsb = min3(end_fsb - offset_fsb, XFS_MAX_BMBT_EXTLEN,
>  			 XFS_B_TO_FSB(mp, 1024 * PAGE_SIZE));
>  
> +	/*
> +	 * When zeroing, don't allocate blocks for holes as they are already
> +	 * zeroes, but we need to ensure that no extents exist in both the data
> +	 * and COW fork to ensure this really is a hole.

But where is the cow fork check?  iomap_iter initializes srcmap to
IOMAP_HOLE, so if we end up here on an IOMAP_ZERO then we've scanned the
data fork and found no mapping.  But then we jump to out_unlock, which
means we don't actually look at ip->cowfp.

<confused>

--D

> +	 */
> +	if ((flags & IOMAP_ZERO) && srcmap->type == IOMAP_HOLE) {
> +		xfs_hole_to_iomap(ip, iomap, offset_fsb, end_fsb);
> +		goto out_unlock;
> +	}
> +
>  	/*
>  	 * The block reservation is supposed to cover all blocks that the
>  	 * operation could possible write, but there is a nasty corner case
> -- 
> 2.52.0
> 
>