FAILED: patch "[PATCH] xfs: fix various problems in xfs_atomic_write_cow_iomap_begin" failed to apply to 6.17-stable tree

From: "Darrick J. Wong" <[email protected]>

[ Upstream commit 8d7bba1e8314013ecc817a91624104ceb9352ddc ]

I think there are several things wrong with this function:

A) xfs_bmapi_write can return a much larger unwritten mapping than what the caller asked for. We convert part of that range to written, but return the entire written mapping to iomap even though that's inaccurate.

B) The arguments to xfs_reflink_convert_cow_locked are wrong -- an unwritten mapping could be *smaller* than the write range (or even the hole range). In this case, we convert too much file range to written state because we then return a smaller mapping to iomap.

C) It doesn't handle delalloc mappings. This I covered in the patch that I already sent to the list.

D) Reassigning count_fsb to handle the hole means that if the second cmap lookup attempt succeeds (due to racing with someone else) we trim the mapping more than is strictly necessary. The changing meaning of count_fsb makes this harder to notice.

E) The tracepoint is kinda wrong because @length is mutated. That makes it harder to chase the data flows through this function because you can't just grep on the pos/bytecount strings.

F) We don't actually check that the br_state = XFS_EXT_NORM assignment is accurate, i.e that the cow fork actually contains a written mapping for the range we're interested in

G) Somewhat inadequate documentation of why we need to xfs_trim_extent so aggressively in this function.

H) Not sure why xfs_iomap_end_fsb is used here, the vfs already clamped the write range to s_maxbytes.

Fix these issues, and then the atomic writes regressions in generic/760, generic/617, generic/091, generic/263, and generic/521 all go away for me.

Cc [email protected] # v6.16

Fixes bd1d2c21d5d249 ("xfs: add xfs_atomic_write_cow_iomap_begin()")

Signed-off-by Darrick J. Wong <[email protected]>

Reviewed-by John Garry <[email protected]>

Signed-off-by Carlos Maiolino <[email protected]>

Signed-off-by Sasha Levin <[email protected]>

---
 fs/xfs/xfs_iomap.c | 61 +++++++++++++++++++++++++++++++++++++---------
 1 file changed, 50 insertions(+), 11 deletions(-)

fs/xfs/xfs_iomap.c +50 -11

index a4a22975c7cc9..0cf84b25d6567 100644
@@ -1082,6 +1082,29 @@ const struct iomap_ops xfs_zoned_direct_write_iomap_ops = {
 };
 #endif /* CONFIG_XFS_RT */
 
+#ifdef DEBUG
+static void
+xfs_check_atomic_cow_conversion(
+	struct xfs_inode		*ip,
+	xfs_fileoff_t			offset_fsb,
+	xfs_filblks_t			count_fsb,
+	const struct xfs_bmbt_irec	*cmap)
+{
+	struct xfs_iext_cursor		icur;
+	struct xfs_bmbt_irec		cmap2 = { };
+
+	if (xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap2))
+		xfs_trim_extent(&cmap2, offset_fsb, count_fsb);
+
+	ASSERT(cmap2.br_startoff == cmap->br_startoff);
+	ASSERT(cmap2.br_blockcount == cmap->br_blockcount);
+	ASSERT(cmap2.br_startblock == cmap->br_startblock);
+	ASSERT(cmap2.br_state == cmap->br_state);
+}
+#else
+# define xfs_check_atomic_cow_conversion(...)	((void)0)
+#endif
+
 static int
 xfs_atomic_write_cow_iomap_begin(
 	struct inode		*inode,

@@ -1093,9 +1116,10 @@ xfs_atomic_write_cow_iomap_begin(
 {
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct xfs_mount	*mp = ip->i_mount;
-	const xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
-	xfs_fileoff_t		end_fsb = xfs_iomap_end_fsb(mp, offset, length);
-	xfs_filblks_t		count_fsb = end_fsb - offset_fsb;
+	const xfs_fileoff_t	offset_fsb = XFS_B_TO_FSBT(mp, offset);
+	const xfs_fileoff_t	end_fsb = XFS_B_TO_FSB(mp, offset + length);
+	const xfs_filblks_t	count_fsb = end_fsb - offset_fsb;
+	xfs_filblks_t		hole_count_fsb;
 	int			nmaps = 1;
 	xfs_filblks_t		resaligned;
 	struct xfs_bmbt_irec	cmap;

@@ -1134,14 +1158,20 @@ xfs_atomic_write_cow_iomap_begin(
 	if (cmap.br_startoff <= offset_fsb) {
 		if (isnullstartblock(cmap.br_startblock))
 			goto convert_delay;
+
+		/*
+		 * cmap could extend outside the write range due to previous
+		 * speculative preallocations.  We must trim cmap to the write
+		 * range because the cow fork treats written mappings to mean
+		 * "write in progress".
+		 */
 		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
 		goto found;
 	}
 
-	end_fsb = cmap.br_startoff;
-	count_fsb = end_fsb - offset_fsb;
+	hole_count_fsb = cmap.br_startoff - offset_fsb;
 
-	resaligned = xfs_aligned_fsb_count(offset_fsb, count_fsb,
+	resaligned = xfs_aligned_fsb_count(offset_fsb, hole_count_fsb,
 			xfs_get_cowextsz_hint(ip));
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 

@@ -1177,7 +1207,7 @@ xfs_atomic_write_cow_iomap_begin(
 	 * atomic writes to that same range will be aligned (and don't require
 	 * this COW-based method).
 	 */
-	error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
+	error = xfs_bmapi_write(tp, ip, offset_fsb, hole_count_fsb,
 			XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC |
 			XFS_BMAPI_EXTSZALIGN, 0, &cmap, &nmaps);
 	if (error) {

@@ -1190,17 +1220,26 @@ xfs_atomic_write_cow_iomap_begin(
 	if (error)
 		goto out_unlock;
 
+	/*
+	 * cmap could map more blocks than the range we passed into bmapi_write
+	 * because of EXTSZALIGN or adjacent pre-existing unwritten mappings
+	 * that were merged.  Trim cmap to the original write range so that we
+	 * don't convert more than we were asked to do for this write.
+	 */
+	xfs_trim_extent(&cmap, offset_fsb, count_fsb);
+
 found:
 	if (cmap.br_state != XFS_EXT_NORM) {
-		error = xfs_reflink_convert_cow_locked(ip, offset_fsb,
-				count_fsb);
+		error = xfs_reflink_convert_cow_locked(ip, cmap.br_startoff,
+				cmap.br_blockcount);
 		if (error)
 			goto out_unlock;
 		cmap.br_state = XFS_EXT_NORM;
+		xfs_check_atomic_cow_conversion(ip, offset_fsb, count_fsb,
+				&cmap);
 	}
 
-	length = XFS_FSB_TO_B(mp, cmap.br_startoff + cmap.br_blockcount);
-	trace_xfs_iomap_found(ip, offset, length - offset, XFS_COW_FORK, &cmap);
+	trace_xfs_iomap_found(ip, offset, length, XFS_COW_FORK, &cmap);
 	seq = xfs_iomap_inode_sequence(ip, IOMAP_F_SHARED);
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 	return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, IOMAP_F_SHARED, seq);

Signature · 1 line

2.51.0

Quoted 158 lines · Sasha Levin

From: "Darrick J. Wong" <[email protected]>

[ Upstream commit 8d7bba1e8314013ecc817a91624104ceb9352ddc ]

I think there are several things wrong with this function:

A) xfs_bmapi_write can return a much larger unwritten mapping than what the caller asked for. We convert part of that range to written, but return the entire written mapping to iomap even though that's inaccurate.

B) The arguments to xfs_reflink_convert_cow_locked are wrong -- an unwritten mapping could be *smaller* than the write range (or even the hole range). In this case, we convert too much file range to written state because we then return a smaller mapping to iomap.

C) It doesn't handle delalloc mappings. This I covered in the patch that I already sent to the list.

D) Reassigning count_fsb to handle the hole means that if the second cmap lookup attempt succeeds (due to racing with someone else) we trim the mapping more than is strictly necessary. The changing meaning of count_fsb makes this harder to notice.

E) The tracepoint is kinda wrong because @length is mutated. That makes it harder to chase the data flows through this function because you can't just grep on the pos/bytecount strings.

F) We don't actually check that the br_state = XFS_EXT_NORM assignment is accurate, i.e that the cow fork actually contains a written mapping for the range we're interested in

G) Somewhat inadequate documentation of why we need to xfs_trim_extent so aggressively in this function.

H) Not sure why xfs_iomap_end_fsb is used here, the vfs already clamped the write range to s_maxbytes.

Fix these issues, and then the atomic writes regressions in generic/760, generic/617, generic/091, generic/263, and generic/521 all go away for me.

Cc [email protected] # v6.16

Fixes bd1d2c21d5d249 ("xfs: add xfs_atomic_write_cow_iomap_begin()")

Signed-off-by Darrick J. Wong <[email protected]>

Reviewed-by John Garry <[email protected]>

Signed-off-by Carlos Maiolino <[email protected]>

Signed-off-by Sasha Levin <[email protected]>

---
 fs/xfs/xfs_iomap.c | 61 +++++++++++++++++++++++++++++++++++++---------
 1 file changed, 50 insertions(+), 11 deletions(-)

fs/xfs/xfs_iomap.c +49 -9

index a4a22975c7cc9..0cf84b25d6567 100644
@@ -1082,6 +1082,29 @@ const struct iomap_ops xfs_zoned_direct_write_iomap_ops = {
 };
 #endif /* CONFIG_XFS_RT */
 
+#ifdef DEBUG
+static void
+xfs_check_atomic_cow_conversion(
+	struct xfs_inode		*ip,
+	xfs_fileoff_t			offset_fsb,
+	xfs_filblks_t			count_fsb,
+	const struct xfs_bmbt_irec	*cmap)
+{
+	struct xfs_iext_cursor		icur;
+	struct xfs_bmbt_irec		cmap2 = { };
+
+	if (xfs_iext_lookup_extent(ip, ip->i_cowfp, offset_fsb, &icur, &cmap2))
+		xfs_trim_extent(&cmap2, offset_fsb, count_fsb);
+
+	ASSERT(cmap2.br_startoff == cmap->br_startoff);
+	ASSERT(cmap2.br_blockcount == cmap->br_blockcount);
+	ASSERT(cmap2.br_startblock == cmap->br_startblock);
+	ASSERT(cmap2.br_state == cmap->br_state);
+}
+#else
+# define xfs_check_atomic_cow_conversion(...)	((void)0)
+#endif
+
 static int
 xfs_atomic_write_cow_iomap_begin(
 	struct inode		*inode,

@@ -1093,9 +1116,10 @@ xfs_atomic_write_cow_iomap_begin(
 {
 	struct xfs_inode	*ip = XFS_I(inode);
 	struct xfs_mount	*mp = ip->i_mount;
-	const xfs_fileoff_t		offset_fsb = XFS_B_TO_FSBT(mp, offset);
-	xfs_fileoff_t		end_fsb = xfs_iomap_end_fsb(mp, offset, length);
-	xfs_filblks_t		count_fsb = end_fsb - offset_fsb;
+	const xfs_fileoff_t	offset_fsb = XFS_B_TO_FSBT(mp, offset);
+	const xfs_fileoff_t	end_fsb = XFS_B_TO_FSB(mp, offset + length);
+	const xfs_filblks_t	count_fsb = end_fsb - offset_fsb;
+	xfs_filblks_t		hole_count_fsb;
 	int			nmaps = 1;
 	xfs_filblks_t		resaligned;
 	struct xfs_bmbt_irec	cmap;

@@ -1134,14 +1158,20 @@ xfs_atomic_write_cow_iomap_begin(
 	if (cmap.br_startoff <= offset_fsb) {
 		if (isnullstartblock(cmap.br_startblock))
 			goto convert_delay;
+
+		/*
+		 * cmap could extend outside the write range due to previous
+		 * speculative preallocations.  We must trim cmap to the write
+		 * range because the cow fork treats written mappings to mean
+		 * "write in progress".
+		 */
 		xfs_trim_extent(&cmap, offset_fsb, count_fsb);
 		goto found;
 	}
 
-	end_fsb = cmap.br_startoff;
-	count_fsb = end_fsb - offset_fsb;
+	hole_count_fsb = cmap.br_startoff - offset_fsb;
 
-	resaligned = xfs_aligned_fsb_count(offset_fsb, count_fsb,
+	resaligned = xfs_aligned_fsb_count(offset_fsb, hole_count_fsb,
 			xfs_get_cowextsz_hint(ip));
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 

@@ -1177,7 +1207,7 @@ xfs_atomic_write_cow_iomap_begin(
 	 * atomic writes to that same range will be aligned (and don't require
 	 * this COW-based method).
 	 */
-	error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
+	error = xfs_bmapi_write(tp, ip, offset_fsb, hole_count_fsb,
 			XFS_BMAPI_COWFORK | XFS_BMAPI_PREALLOC |
 			XFS_BMAPI_EXTSZALIGN, 0, &cmap, &nmaps);
 	if (error) {

@@ -1190,17 +1220,26 @@ xfs_atomic_write_cow_iomap_begin(
 	if (error)
 		goto out_unlock;
 
+	/*
+	 * cmap could map more blocks than the range we passed into bmapi_write
+	 * because of EXTSZALIGN or adjacent pre-existing unwritten mappings
+	 * that were merged.  Trim cmap to the original write range so that we
+	 * don't convert more than we were asked to do for this write.
+	 */
+	xfs_trim_extent(&cmap, offset_fsb, count_fsb);
+
 found:
 	if (cmap.br_state != XFS_EXT_NORM) {
-		error = xfs_reflink_convert_cow_locked(ip, offset_fsb,
-				count_fsb);
+		error = xfs_reflink_convert_cow_locked(ip, cmap.br_startoff,
+				cmap.br_blockcount);
 		if (error)
 			goto out_unlock;
 		cmap.br_state = XFS_EXT_NORM;
+		xfs_check_atomic_cow_conversion(ip, offset_fsb, count_fsb,
+				&cmap);
 	}
 

Quoted 10 lines

fs/xfs/xfs_iomap.c +1 -3

-	length = XFS_FSB_TO_B(mp, cmap.br_startoff + cmap.br_blockcount);
-	trace_xfs_iomap_found(ip, offset, length - offset, XFS_COW_FORK, &cmap);
+	trace_xfs_iomap_found(ip, offset, length, XFS_COW_FORK, &cmap);
 	seq = xfs_iomap_inode_sequence(ip, IOMAP_F_SHARED);
 	xfs_iunlock(ip, XFS_ILOCK_EXCL);
 	return xfs_bmbt_to_iomap(ip, iomap, &cmap, flags, IOMAP_F_SHARED, seq);
--

2.51.0

Also did not apply to 6.17.y :(

From: "Darrick J. Wong" <[email protected]>

[ Upstream commit 8d54eacd82a0623a963e0c150ad3b02970638b0d ]

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 [email protected] # v6.16

Fixes bd1d2c21d5d249 ("xfs: add xfs_atomic_write_cow_iomap_begin()")

Signed-off-by Darrick J. Wong <[email protected]>

Reviewed-by John Garry <[email protected]>

Signed-off-by Carlos Maiolino <[email protected]>

Stable-dep-of: 8d7bba1e8314 ("xfs: fix various problems in xfs_atomic_write_cow_iomap_begin")

Signed-off-by Sasha Levin <[email protected]>

---
 fs/xfs/xfs_iomap.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

fs/xfs/xfs_iomap.c +19 -2

index 2a74f29573410..a4a22975c7cc9 100644
@@ -1121,7 +1121,7 @@ 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) {

@@ -1132,6 +1132,8 @@ 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;
 	}

@@ -1160,8 +1162,10 @@ 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;
 	}
 

@@ -1201,6 +1205,19 @@ 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;

Signature · 1 line

2.51.0