xfs: fix delalloc write failures in software-provided atomic writes

With the 20 Oct 2025 release of fstests, generic/521 fails for me on
regular (aka non-block-atomic-writes) storage:

QA output created by 521
dowrite: write: Input/output error
LOG DUMP (8553 total operations):
1(  1 mod 256): SKIPPED (no operation)
2(  2 mod 256): WRITE    0x7e000 thru 0x8dfff	(0x10000 bytes) HOLE
3(  3 mod 256): READ     0x69000 thru 0x79fff	(0x11000 bytes)
4(  4 mod 256): FALLOC   0x53c38 thru 0x5e853	(0xac1b bytes) INTERIOR
5(  5 mod 256): COPY 0x55000 thru 0x59fff	(0x5000 bytes) to 0x25000 thru 0x29fff
6(  6 mod 256): WRITE    0x74000 thru 0x88fff	(0x15000 bytes)
7(  7 mod 256): ZERO     0xedb1 thru 0x11693	(0x28e3 bytes)

with a warning in dmesg from iomap about XFS trying to give it a
delalloc mapping for a directio write.  Fix the software atomic write
iomap_begin code to convert the reservation into a written mapping.
This doesn't fix the data corruption problems reported by generic/760,
but it's a start.

Cc: stable@vger.kernel.org # v6.16
Fixes: bd1d2c2 ("xfs: add xfs_atomic_write_cow_iomap_begin()")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

Author: Darrick J. Wong

fs/xfs/xfs_iomap.c

@@ -1130,7 +1130,7 @@ xfs_atomic_write_cow_iomap_begin(
 		return -EAGAIN;
 
 	trace_xfs_iomap_atomic_write_cow(ip, offset, length);
-
+retry:
 	xfs_ilock(ip, XFS_ILOCK_EXCL);
 
 	if (!ip->i_cowfp) {
@@ -1141,6 +1141,8 @@ xfs_atomic_write_cow_iomap_begin(
 	if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap))
 		cmap.br_startoff = end_fsb;
 	if (cmap.br_startoff <= offset_fsb) {
+		if (isnullstartblock(cmap.br_startblock))
+			goto convert_delay;
 		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
 		goto found;
 	}
@@ -1169,8 +1171,10 @@ xfs_atomic_write_cow_iomap_begin(
 	if (!xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap))
 		cmap.br_startoff = end_fsb;
 	if (cmap.br_startoff <= offset_fsb) {
-		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
 		xfs_trans_cancel(tp);
+		if (isnullstartblock(cmap.br_startblock))
+			goto convert_delay;
+		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
 		goto found;
 	}
 
@@ -1210,6 +1214,19 @@ xfs_atomic_write_cow_iomap_begin(
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 	return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, IOMAP_F_SHARED, seq);
 
+convert_delay:
+	xfs_iunlock(ip, XFS_ILOCK_EXCL);
+	error = xfs_bmapi_convert_delalloc(ip, XFS_COW_FORK, offset, iomap,
+			NULL);
+	if (error)
+		return error;
+
+	/*
+	 * Try the lookup again, because the delalloc conversion might have
+	 * turned the COW mapping into unwritten, but we need it to be in
+	 * written state.
+	 */
+	goto retry;
 out_unlock:
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 	return error;