[PATCH v3 0/3] add FALLOC_FL_WRITE_ZEROES support to xfs

[PATCH v3 0/3] add FALLOC_FL_WRITE_ZEROES support to xfs

[Pankaj Raghav]

The benefits of FALLOC_FL_WRITE_ZEROES was already discussed as a part
of Zhang Yi's initial patches[1]. Postgres developer Andres also
mentioned they would like to use this feature in Postgres [2].

This series has the changes proposed by Dave [3].

I tested the changes with fsstress and fsx based on the xfstests patch I
sent recently to test this flag[4]. generic/363 helped me debug the
crash I noticed when I did the initial implementation[3].

Changes since v2:
- Add allow_write_zeroes to xfs_global so that we can enable this
  feature independent of the HW underneath.

Changes since v1 [5.1 5.2]:
- Added a new function xfs_bmap_alloc_or_convert_range() based on Dave's
  feedback.
- Changed the xfs_falloc_write_zeroes to use
  xfs_bmap_alloc_or_convert_range() instead of doing prealloc and
  convert approach.

[1] https://lore.kernel.org/linux-fsdevel/20250619111806.3546162-1-yi.zhang@huaweicloud.com/
[2] https://lore.kernel.org/linux-fsdevel/20260217055103.GA6174@lst.de/T/#m7935b9bab32bb5ff372507f84803b8753ad1c814
[3] https://lore.kernel.org/linux-xfs/6i2jvzn3lyugjlbgmjzpped3gogzyqv5mpe2uqaifz4vjpaega@pomzoq7ley77/
[4] https://lore.kernel.org/linux-xfs/20260312195308.738189-1-p.raghav@samsung.com/
[5.1] https://lore.kernel.org/linux-xfs/20260309180708.427553-2-lukas@herbolt.com/
[5.2] https://lore.kernel.org/linux-xfs/abC1LvRElctaHPe5@dread/

Lukas Herbolt (1):
  xfs: add a sysfs entry to enable write zeroes even if HW does not
    support it

Pankaj Raghav (2):
  xfs: add xfs_bmap_alloc_or_convert_range function
  xfs: add support for FALLOC_FL_WRITE_ZEROES

 fs/xfs/xfs_bmap_util.c | 165 +++++++++++++++++++++++++++++++++++++++++
 fs/xfs/xfs_bmap_util.h |   3 +
 fs/xfs/xfs_file.c      |  57 +++++++++++++-
 fs/xfs/xfs_iomap.c     | 113 +---------------------------
 fs/xfs/xfs_iomap.h     |   1 +
 fs/xfs/xfs_sysctl.h    |   1 +
 fs/xfs/xfs_sysfs.c     |  25 +++++++
 7 files changed, 254 insertions(+), 111 deletions(-)


base-commit: c43493d3df4aa2a4b39a058f965247dfbd4a708f
-- 
2.51.2

[PATCH v3 1/3] xfs: add xfs_bmap_alloc_or_convert_range function

[Pankaj Raghav]

Add xfs_bmap_alloc_or_convert_range() that can either allocate a range
and/or convert unwritten extents to written extents.

This function is based on xfs_iomap_write_unwritten() but we add an
extra flag parameter. Only XFS_BMAPI_CONVERT and/or XFS_BMAPI_ZERO is
accepted as flags to this function. This function also additionally
accounts while starting the transaction for the blocks that might
be created because of XFS_BMAPI_ZERO.

This is done as a preparation to add FALLOC_FL_WRITE_ZEROES flag.

xfs_iomap_write_unwritten() function will now just call
xfs_bmap_alloc_or_convert_range() with flag set to XFS_BMAPI_CONVERT.

Suggested-by: Dave Chinner <dgc@kernel.org>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
 fs/xfs/xfs_bmap_util.c | 165 +++++++++++++++++++++++++++++++++++++++++
 fs/xfs/xfs_bmap_util.h |   3 +
 fs/xfs/xfs_iomap.c     | 113 +---------------------------
 fs/xfs/xfs_iomap.h     |   1 +
 4 files changed, 172 insertions(+), 110 deletions(-)

diff --git a/fs/xfs/xfs_bmap_util.c b/fs/xfs/xfs_bmap_util.c
index 0ab00615f1ad..5a6abca12982 100644
--- a/fs/xfs/xfs_bmap_util.c
+++ b/fs/xfs/xfs_bmap_util.c
@@ -20,6 +20,7 @@
 #include "xfs_bmap.h"
 #include "xfs_bmap_util.h"
 #include "xfs_bmap_btree.h"
+#include "xfs_health.h"
 #include "xfs_rtalloc.h"
 #include "xfs_error.h"
 #include "xfs_quota.h"
@@ -642,6 +643,170 @@ xfs_free_eofblocks(
 	return error;
 }
 
+/*
+ * This function is used to allocate written extents over holes
+ * and/or convert unwritten extents to written extents based on the
+ * @flags passed to it.
+ *
+ * If @flags is zero, it will allocate written extents for holes and
+ * delalloc extents across the range.
+ *
+ * If XFS_BMAPI_CONVERT is specified in @flags, then it will also do
+ * conversion of unwritten extents in the range to written extents.
+ *
+ * If XFS_BMAPI_ZERO is specified in @flags, then both newly
+ * allocated extents and converted unwritten extents will be
+ * initialised to contain zeroes.
+ *
+ * If @update_isize is true, then if the range we are operating on
+ * extends beyond the current EOF, extend i_size to offset+len
+ * incrementally as extents in the range are allocated/converted.
+ */
+int
+xfs_bmap_alloc_or_convert_range(
+	struct xfs_inode	*ip,
+	xfs_off_t		offset,
+	xfs_off_t		count,
+	uint32_t		flags,
+	bool			update_isize)
+{
+	xfs_mount_t	*mp = ip->i_mount;
+	xfs_fileoff_t	offset_fsb;
+	xfs_filblks_t	count_fsb;
+	xfs_filblks_t	numblks_fsb;
+	int		nimaps;
+	xfs_trans_t	*tp;
+	xfs_bmbt_irec_t imap;
+	struct inode	*inode = VFS_I(ip);
+	xfs_fsize_t	i_size;
+	int		error;
+
+	ASSERT((flags & ~(XFS_BMAPI_ZERO | XFS_BMAPI_CONVERT)) == 0);
+
+	offset_fsb = XFS_B_TO_FSBT(mp, offset);
+	count_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + count);
+	count_fsb = (xfs_filblks_t)(count_fsb - offset_fsb);
+
+	/* Attach dquots so that bmbt splits are accounted correctly. */
+	error = xfs_qm_dqattach(ip);
+	if (error)
+		return error;
+
+	do {
+
+		uint	resblks, dblocks;
+		uint	bmapi_total = 0;
+		int bmapi_error;
+
+		if (flags == XFS_BMAPI_CONVERT) {
+			/*
+			 * Pure unwritten conversion. No data blocks needed.
+			 * Reserve enough for two full b-tree splits.
+			 */
+			resblks = 0;
+			dblocks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;
+			bmapi_total = dblocks;
+		} else {
+			/*
+			 * We might allocate data blocks (needs resblks + 1 split) or
+			 * convert an unwritten extent (needs 0 data blocks + 2 splits).
+			 * Ensure we have enough block reservation for the worst case.
+			 */
+			resblks = XFS_FILBLKS_MIN(count_fsb, XFS_MAX_BMBT_EXTLEN);
+			dblocks = XFS_DIOSTRAT_SPACE_RES(mp, resblks);
+			dblocks += (XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1);
+			bmapi_total = 0;
+		}
+
+		/*
+		 * Set up a transaction to convert the range of extents
+		 * based on the flags. Do allocations in a loop until
+		 * we have covered the range passed in.
+		 *
+		 * Note that we can't risk to recursing back into the filesystem
+		 * here as we might be asked to write out the same inode that we
+		 * complete here and might deadlock on the iolock.
+		 */
+		error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write, dblocks,
+				0, true, &tp);
+		if (error)
+			return error;
+
+		if (flags & XFS_BMAPI_CONVERT)
+			error = xfs_iext_count_extend(tp, ip, XFS_DATA_FORK,
+					XFS_IEXT_WRITE_UNWRITTEN_CNT);
+		else
+			error = xfs_iext_count_extend(tp, ip, XFS_DATA_FORK,
+					XFS_IEXT_ADD_NOSPLIT_CNT);
+
+		if (error)
+			goto error_on_bmapi_transaction;
+
+		nimaps = 1;
+		error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
+					flags, bmapi_total, &imap, &nimaps);
+		bmapi_error = error;
+
+		if (error) {
+			if (error != -ENOSR)
+				goto error_on_bmapi_transaction;
+			/*
+			 * Keep searching until we get one contiguous
+			 * extent if we get ENOSR
+			 */
+			error = 0;
+		} else {
+			/*
+			 * Log the updated inode size as we go.  We have to be careful
+			 * to only log it up to the actual write offset if it is
+			 * halfway into a block.
+			 */
+			i_size = XFS_FSB_TO_B(mp, offset_fsb + count_fsb);
+			if (i_size > offset + count)
+				i_size = offset + count;
+			if (update_isize && i_size > i_size_read(inode))
+				i_size_write(inode, i_size);
+			i_size = xfs_new_eof(ip, i_size);
+			if (i_size) {
+				ip->i_disk_size = i_size;
+				xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
+			}
+
+		}
+
+		error = xfs_trans_commit(tp);
+		xfs_iunlock(ip, XFS_ILOCK_EXCL);
+
+		if (!bmapi_error) {
+			if (unlikely(!xfs_valid_startblock(ip, imap.br_startblock))) {
+				xfs_bmap_mark_sick(ip, XFS_DATA_FORK);
+				return xfs_alert_fsblock_zero(ip, &imap);
+			}
+			if ((numblks_fsb = imap.br_blockcount) == 0) {
+				/*
+				 * The numblks_fsb value should always get
+				 * smaller, otherwise the loop is stuck.
+				 */
+				ASSERT(imap.br_blockcount);
+				break;
+			}
+			offset_fsb += numblks_fsb;
+			count_fsb -= numblks_fsb;
+		}
+
+		if (error)
+			return error;
+
+	} while (count_fsb > 0);
+
+	return 0;
+
+error_on_bmapi_transaction:
+	xfs_trans_cancel(tp);
+	xfs_iunlock(ip, XFS_ILOCK_EXCL);
+	return error;
+}
+
 int
 xfs_alloc_file_space(
 	struct xfs_inode	*ip,
diff --git a/fs/xfs/xfs_bmap_util.h b/fs/xfs/xfs_bmap_util.h
index c477b3361630..0bf84130f49d 100644
--- a/fs/xfs/xfs_bmap_util.h
+++ b/fs/xfs/xfs_bmap_util.h
@@ -57,7 +57,10 @@ int	xfs_bmap_last_extent(struct xfs_trans *tp, struct xfs_inode *ip,
 /* preallocation and hole punch interface */
 int	xfs_alloc_file_space(struct xfs_inode *ip, xfs_off_t offset,
 		xfs_off_t len);
+int	xfs_bmap_alloc_or_convert_range(struct xfs_inode *ip, xfs_off_t offset,
+		xfs_off_t count, uint32_t flags, bool update_isize);
 int	xfs_free_file_space(struct xfs_inode *ip, xfs_off_t offset,
+
 		xfs_off_t len, struct xfs_zone_alloc_ctx *ac);
 int	xfs_collapse_file_space(struct xfs_inode *, xfs_off_t offset,
 		xfs_off_t len, struct xfs_zone_alloc_ctx *ac);
diff --git a/fs/xfs/xfs_iomap.c b/fs/xfs/xfs_iomap.c
index f20a02f49ed9..4524907f91e3 100644
--- a/fs/xfs/xfs_iomap.c
+++ b/fs/xfs/xfs_iomap.c
@@ -36,7 +36,7 @@
 #define XFS_ALLOC_ALIGN(mp, off) \
 	(((off) >> mp->m_allocsize_log) << mp->m_allocsize_log)
 
-static int
+int
 xfs_alert_fsblock_zero(
 	xfs_inode_t	*ip,
 	xfs_bmbt_irec_t	*imap)
@@ -616,115 +616,8 @@ xfs_iomap_write_unwritten(
 	xfs_off_t	count,
 	bool		update_isize)
 {
-	xfs_mount_t	*mp = ip->i_mount;
-	xfs_fileoff_t	offset_fsb;
-	xfs_filblks_t	count_fsb;
-	xfs_filblks_t	numblks_fsb;
-	int		nimaps;
-	xfs_trans_t	*tp;
-	xfs_bmbt_irec_t imap;
-	struct inode	*inode = VFS_I(ip);
-	xfs_fsize_t	i_size;
-	uint		resblks;
-	int		error;
-
-	trace_xfs_unwritten_convert(ip, offset, count);
-
-	offset_fsb = XFS_B_TO_FSBT(mp, offset);
-	count_fsb = XFS_B_TO_FSB(mp, (xfs_ufsize_t)offset + count);
-	count_fsb = (xfs_filblks_t)(count_fsb - offset_fsb);
-
-	/*
-	 * Reserve enough blocks in this transaction for two complete extent
-	 * btree splits.  We may be converting the middle part of an unwritten
-	 * extent and in this case we will insert two new extents in the btree
-	 * each of which could cause a full split.
-	 *
-	 * This reservation amount will be used in the first call to
-	 * xfs_bmbt_split() to select an AG with enough space to satisfy the
-	 * rest of the operation.
-	 */
-	resblks = XFS_DIOSTRAT_SPACE_RES(mp, 0) << 1;
-
-	/* Attach dquots so that bmbt splits are accounted correctly. */
-	error = xfs_qm_dqattach(ip);
-	if (error)
-		return error;
-
-	do {
-		/*
-		 * Set up a transaction to convert the range of extents
-		 * from unwritten to real. Do allocations in a loop until
-		 * we have covered the range passed in.
-		 *
-		 * Note that we can't risk to recursing back into the filesystem
-		 * here as we might be asked to write out the same inode that we
-		 * complete here and might deadlock on the iolock.
-		 */
-		error = xfs_trans_alloc_inode(ip, &M_RES(mp)->tr_write, resblks,
-				0, true, &tp);
-		if (error)
-			return error;
-
-		error = xfs_iext_count_extend(tp, ip, XFS_DATA_FORK,
-				XFS_IEXT_WRITE_UNWRITTEN_CNT);
-		if (error)
-			goto error_on_bmapi_transaction;
-
-		/*
-		 * Modify the unwritten extent state of the buffer.
-		 */
-		nimaps = 1;
-		error = xfs_bmapi_write(tp, ip, offset_fsb, count_fsb,
-					XFS_BMAPI_CONVERT, resblks, &imap,
-					&nimaps);
-		if (error)
-			goto error_on_bmapi_transaction;
-
-		/*
-		 * Log the updated inode size as we go.  We have to be careful
-		 * to only log it up to the actual write offset if it is
-		 * halfway into a block.
-		 */
-		i_size = XFS_FSB_TO_B(mp, offset_fsb + count_fsb);
-		if (i_size > offset + count)
-			i_size = offset + count;
-		if (update_isize && i_size > i_size_read(inode))
-			i_size_write(inode, i_size);
-		i_size = xfs_new_eof(ip, i_size);
-		if (i_size) {
-			ip->i_disk_size = i_size;
-			xfs_trans_log_inode(tp, ip, XFS_ILOG_CORE);
-		}
-
-		error = xfs_trans_commit(tp);
-		xfs_iunlock(ip, XFS_ILOCK_EXCL);
-		if (error)
-			return error;
-
-		if (unlikely(!xfs_valid_startblock(ip, imap.br_startblock))) {
-			xfs_bmap_mark_sick(ip, XFS_DATA_FORK);
-			return xfs_alert_fsblock_zero(ip, &imap);
-		}
-
-		if ((numblks_fsb = imap.br_blockcount) == 0) {
-			/*
-			 * The numblks_fsb value should always get
-			 * smaller, otherwise the loop is stuck.
-			 */
-			ASSERT(imap.br_blockcount);
-			break;
-		}
-		offset_fsb += numblks_fsb;
-		count_fsb -= numblks_fsb;
-	} while (count_fsb > 0);
-
-	return 0;
-
-error_on_bmapi_transaction:
-	xfs_trans_cancel(tp);
-	xfs_iunlock(ip, XFS_ILOCK_EXCL);
-	return error;
+	return xfs_bmap_alloc_or_convert_range(ip, offset, count,
+					XFS_BMAPI_CONVERT, update_isize);
 }
 
 static inline bool
diff --git a/fs/xfs/xfs_iomap.h b/fs/xfs/xfs_iomap.h
index ebcce7d49446..d59428277376 100644
--- a/fs/xfs/xfs_iomap.h
+++ b/fs/xfs/xfs_iomap.h
@@ -16,6 +16,7 @@ int xfs_iomap_write_direct(struct xfs_inode *ip, xfs_fileoff_t offset_fsb,
 		xfs_fileoff_t count_fsb, unsigned int flags,
 		struct xfs_bmbt_irec *imap, u64 *sequence);
 int xfs_iomap_write_unwritten(struct xfs_inode *, xfs_off_t, xfs_off_t, bool);
+int xfs_alert_fsblock_zero(struct xfs_inode *ip, struct xfs_bmbt_irec *imap);
 xfs_fileoff_t xfs_iomap_eof_align_last_fsb(struct xfs_inode *ip,
 		xfs_fileoff_t end_fsb);
 
-- 
2.51.2

[PATCH v3 2/3] xfs: add support for FALLOC_FL_WRITE_ZEROES

[Pankaj Raghav]

If the underlying block device supports the unmap write zeroes
operation, this flag allows users to quickly preallocate a file with
written extents that contain zeroes. This is beneficial for subsequent
overwrites as it prevents the need for unwritten-to-written extent
conversions, thereby significantly reducing metadata updates and journal
I/O overhead, improving overwrite performance.

Co-developed-by: Lukas Herbolt <lukas@herbolt.com>
Signed-off-by: Lukas Herbolt <lukas@herbolt.com>
Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
---
I would like to get feedback on calling xfs_zero_range in
xfs_falloc_write_zeroes function. It gets the job done but I am
wondering if there is a better of handling zeroing out between old EOF
and new offset.

 fs/xfs/xfs_file.c | 56 ++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 55 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 845a97c9b063..99a02982154a 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1368,6 +1368,57 @@ xfs_falloc_force_zero(
 	return XFS_TEST_ERROR(ip->i_mount, XFS_ERRTAG_FORCE_ZERO_RANGE);
 }
 
+static int
+xfs_falloc_write_zeroes(
+	struct file		*file,
+	int			mode,
+	loff_t			offset,
+	loff_t			len,
+	struct xfs_zone_alloc_ctx *ac)
+{
+	struct inode		*inode = file_inode(file);
+	struct xfs_inode	*ip = XFS_I(inode);
+	loff_t			new_size = 0;
+	loff_t			old_size = XFS_ISIZE(ip);
+	int			error;
+	unsigned int		blksize = i_blocksize(inode);
+	loff_t			offset_aligned = round_down(offset, blksize);
+	bool			did_zero;
+
+	if (xfs_is_always_cow_inode(ip) ||
+	    !bdev_write_zeroes_unmap_sectors(
+		    xfs_inode_buftarg(XFS_I(inode))->bt_bdev))
+		return -EOPNOTSUPP;
+
+	error = xfs_falloc_newsize(file, mode, offset, len, &new_size);
+	if (error)
+		return error;
+
+	error = xfs_free_file_space(ip, offset, len, ac);
+	if (error)
+		return error;
+
+	/*
+	 * Zero the tail of the old EOF block and any space up to the new
+	 * offset.
+	 * In the usual truncate path, xfs_falloc_setsize takes care of
+	 * zeroing those blocks.
+	 */
+	if (offset_aligned > old_size)
+		error = xfs_zero_range(ip, old_size, offset_aligned - old_size,
+				NULL, &did_zero);
+	if (error)
+		return error;
+
+	error = xfs_bmap_alloc_or_convert_range(ip, offset, len,
+			XFS_BMAPI_CONVERT | XFS_BMAPI_ZERO,
+			new_size ? true : false);
+	if (error)
+		return error;
+
+	return error;
+}
+
 /*
  * Punch a hole and prealloc the range.  We use a hole punch rather than
  * unwritten extent conversion for two reasons:
@@ -1470,7 +1521,7 @@ xfs_falloc_allocate_range(
 		(FALLOC_FL_ALLOCATE_RANGE | FALLOC_FL_KEEP_SIZE |	\
 		 FALLOC_FL_PUNCH_HOLE |	FALLOC_FL_COLLAPSE_RANGE |	\
 		 FALLOC_FL_ZERO_RANGE |	FALLOC_FL_INSERT_RANGE |	\
-		 FALLOC_FL_UNSHARE_RANGE)
+		 FALLOC_FL_UNSHARE_RANGE | FALLOC_FL_WRITE_ZEROES)
 
 STATIC long
 __xfs_file_fallocate(
@@ -1522,6 +1573,9 @@ __xfs_file_fallocate(
 	case FALLOC_FL_ALLOCATE_RANGE:
 		error = xfs_falloc_allocate_range(file, mode, offset, len);
 		break;
+	case FALLOC_FL_WRITE_ZEROES:
+		error = xfs_falloc_write_zeroes(file, mode, offset, len, ac);
+		break;
 	default:
 		error = -EOPNOTSUPP;
 		break;
-- 
2.51.2

[PATCH v3 3/3] xfs: add a sysfs entry to enable write zeroes even if HW does not support it

[Pankaj Raghav]

From: Lukas Herbolt <lukas@herbolt.com>

Add a xfs sysfs entry that will not return ENOTSUPP for
FALLOC_FL_WRITE_ZEROES flag in fallocate even if the hardware does not
support deallocate on write zeroes. This is useful for testing this
feature independent of the HW underneath.

Signed-off-by: Pankaj Raghav <p.raghav@samsung.com>
Signed-off-by: Lukas Herbolt <lukas@herbolt.com>
---
 fs/xfs/xfs_file.c   |  3 ++-
 fs/xfs/xfs_sysctl.h |  1 +
 fs/xfs/xfs_sysfs.c  | 25 +++++++++++++++++++++++++
 3 files changed, 28 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 99a02982154a..d70959a855db 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -1388,7 +1388,8 @@ xfs_falloc_write_zeroes(
 	if (xfs_is_always_cow_inode(ip) ||
 	    !bdev_write_zeroes_unmap_sectors(
 		    xfs_inode_buftarg(XFS_I(inode))->bt_bdev))
-		return -EOPNOTSUPP;
+		if (!xfs_globals.allow_write_zero)
+			return -EOPNOTSUPP;
 
 	error = xfs_falloc_newsize(file, mode, offset, len, &new_size);
 	if (error)
diff --git a/fs/xfs/xfs_sysctl.h b/fs/xfs/xfs_sysctl.h
index ed9d896079c1..ef46cfdfae43 100644
--- a/fs/xfs/xfs_sysctl.h
+++ b/fs/xfs/xfs_sysctl.h
@@ -86,6 +86,7 @@ struct xfs_globals {
 	int	mount_delay;		/* mount setup delay (secs) */
 	bool	bug_on_assert;		/* BUG() the kernel on assert failure */
 	bool	always_cow;		/* use COW fork for all overwrites */
+	bool    allow_write_zero;       /* Allow WRITE_ZERO on any HW */
 };
 extern struct xfs_globals	xfs_globals;
 
diff --git a/fs/xfs/xfs_sysfs.c b/fs/xfs/xfs_sysfs.c
index 676777064c2d..ed35f5919fbd 100644
--- a/fs/xfs/xfs_sysfs.c
+++ b/fs/xfs/xfs_sysfs.c
@@ -315,6 +315,30 @@ bload_node_slack_show(
 }
 XFS_SYSFS_ATTR_RW(bload_node_slack);
 
+static ssize_t
+allow_write_zero_store(
+       struct kobject  *kobject,
+       const char      *buf,
+       size_t          count)
+{
+       ssize_t         ret;
+
+       ret = kstrtobool(buf, &xfs_globals.allow_write_zero);
+       if (ret < 0)
+               return ret;
+       return count;
+}
+
+static ssize_t
+allow_write_zero_show(
+       struct kobject  *kobject,
+       char            *buf)
+{
+       return sysfs_emit(buf, "%d\n", xfs_globals.allow_write_zero);
+}
+XFS_SYSFS_ATTR_RW(allow_write_zero);
+
+
 static struct attribute *xfs_dbg_attrs[] = {
 	ATTR_LIST(bug_on_assert),
 	ATTR_LIST(log_recovery_delay),
@@ -324,6 +348,7 @@ static struct attribute *xfs_dbg_attrs[] = {
 	ATTR_LIST(larp),
 	ATTR_LIST(bload_leaf_slack),
 	ATTR_LIST(bload_node_slack),
+	ATTR_LIST(allow_write_zero),
 	NULL,
 };
 ATTRIBUTE_GROUPS(xfs_dbg);
-- 
2.51.2

Re: [PATCH v3 1/3] xfs: add xfs_bmap_alloc_or_convert_range function

[Christoph Hellwig]

> +/*
> + * This function is used to allocate written extents over holes
> + * and/or convert unwritten extents to written extents based on the
> + * @flags passed to it.

You can and should use up all 80 characters for comments.

> + */
> +int
> +xfs_bmap_alloc_or_convert_range(
> +	struct xfs_inode	*ip,
> +	xfs_off_t		offset,
> +	xfs_off_t		count,
> +	uint32_t		flags,
> +	bool			update_isize)
> +{

Please keep this function in the same please as the generalized
version it was base on so that git diff/show shows what changed.
If you have to move it around for later patches do that in
an otherwise no-op prep patch.

> +		} else {
> +			/*
> +			 * We might allocate data blocks (needs resblks + 1 split) or
> +			 * convert an unwritten extent (needs 0 data blocks + 2 splits).
> +			 * Ensure we have enough block reservation for the worst case.
> +			 */
> +			resblks = XFS_FILBLKS_MIN(count_fsb, XFS_MAX_BMBT_EXTLEN);

Please avoid the overly long lines.

Re: [PATCH v3 3/3] xfs: add a sysfs entry to enable write zeroes even if HW does not support it

[Christoph Hellwig]

On Mon, Apr 20, 2026 at 12:55:39PM +0200, Pankaj Raghav wrote:
> From: Lukas Herbolt <lukas@herbolt.com>
> 
> Add a xfs sysfs entry that will not return ENOTSUPP for
> FALLOC_FL_WRITE_ZEROES flag in fallocate even if the hardware does not
> support deallocate on write zeroes. This is useful for testing this
> feature independent of the HW underneath.

I don't think it is.  We can easily test this using a loop or scsi_debug
device.  And in fact should by adding an xfstests that uses such
synthetic devices.

Re: [PATCH v3 3/3] xfs: add a sysfs entry to enable write zeroes even if HW does not support it

[Pankaj Raghav]

On 4/24/26 15:52, Christoph Hellwig wrote:
> On Mon, Apr 20, 2026 at 12:55:39PM +0200, Pankaj Raghav wrote:
>> From: Lukas Herbolt <lukas@herbolt.com>
>>
>> Add a xfs sysfs entry that will not return ENOTSUPP for
>> FALLOC_FL_WRITE_ZEROES flag in fallocate even if the hardware does not
>> support deallocate on write zeroes. This is useful for testing this
>> feature independent of the HW underneath.
> 
> I don't think it is.  We can easily test this using a loop or scsi_debug
> device.  And in fact should by adding an xfstests that uses such
> synthetic devices.
> 

This makes sense. Also this avoids adding hardware specific knobs in the filesystem.

--
Pankaj

Re: [PATCH v3 1/3] xfs: add xfs_bmap_alloc_or_convert_range function

[Pankaj Raghav]

On 4/24/26 15:52, Christoph Hellwig wrote:
>> +/*
>> + * This function is used to allocate written extents over holes
>> + * and/or convert unwritten extents to written extents based on the
>> + * @flags passed to it.
> 
> You can and should use up all 80 characters for comments.
> 
>> + */
>> +int
>> +xfs_bmap_alloc_or_convert_range(
>> +	struct xfs_inode	*ip,
>> +	xfs_off_t		offset,
>> +	xfs_off_t		count,
>> +	uint32_t		flags,
>> +	bool			update_isize)
>> +{
> 
> Please keep this function in the same please as the generalized
> version it was base on so that git diff/show shows what changed.
> If you have to move it around for later patches do that in
> an otherwise no-op prep patch.
> 

So IIUC, you are suggesting to add this function in xfs_iomap.c itself and move it to
bmap_util.c in an another prep patch.

>> +		} else {
>> +			/*
>> +			 * We might allocate data blocks (needs resblks + 1 split) or
>> +			 * convert an unwritten extent (needs 0 data blocks + 2 splits).
>> +			 * Ensure we have enough block reservation for the worst case.
>> +			 */
>> +			resblks = XFS_FILBLKS_MIN(count_fsb, XFS_MAX_BMBT_EXTLEN);
> 
> Please avoid the overly long lines.
> 

I will change them all in the next version. Thanks for your comments.

--
Pankaj