[PATCH v6 09/12] xfs: commit CoW-based atomic writes atomically

John Garry
2025-04-08T10:45:05Z

When completing a CoW-based write, each extent range mapping update is
covered by a separate transaction.

For a CoW-based atomic write, all mappings must be changed at once, so
change to use a single transaction.

Note that there is a limit on the amount of log intent items which can be
fit into a single transaction, but this is being ignored for now since
the count of items for a typical atomic write would be much less than is
typically supported. A typical atomic write would be expected to be 64KB
or less, which means only 16 possible extents unmaps, which is quite
small.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
fs/xfs/xfs_file.c | 5 +++-
fs/xfs/xfs_reflink.c | 56 ++++++++++++++++++++++++++++++++++++++++++++
fs/xfs/xfs_reflink.h | 2 ++
3 files changed, 62 insertions(+), 1 deletion(-)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 1302783a7157..ba4b02abc6e4 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -576,7 +576,10 @@ xfs_dio_write_end_io(
nofs_flag = memalloc_nofs_save();

if (flags & IOMAP_DIO_COW) {
- error = xfs_reflink_end_cow(ip, offset, size);
+ if (iocb->ki_flags & IOCB_ATOMIC)
+ error = xfs_reflink_end_atomic_cow(ip, offset, size);
+ else
+ error = xfs_reflink_end_cow(ip, offset, size);
if (error)
goto out;
}
diff --git a/fs/xfs/xfs_reflink.c b/fs/xfs/xfs_reflink.c
index f5d338916098..01f1930fdde6 100644
--- a/fs/xfs/xfs_reflink.c
+++ b/fs/xfs/xfs_reflink.c
@@ -984,6 +984,62 @@ xfs_reflink_end_cow(
return error;
}

+/*
+ * Fully remap all of the file's data fork at once, which is the critical part
+ * in achieving atomic behaviour.
+ * The regular CoW end path does not use function as to keep the block
+ * reservation per transaction as low as possible.
+ */
+int
+xfs_reflink_end_atomic_cow(
+ struct xfs_inode *ip,
+ xfs_off_t offset,
+ xfs_off_t count)
+{
+ xfs_fileoff_t offset_fsb;
+ xfs_fileoff_t end_fsb;
+ int error = 0;
+ struct xfs_mount *mp = ip->i_mount;
+ struct xfs_trans *tp;
+ unsigned int resblks;
+
+ trace_xfs_reflink_end_cow(ip, offset, count);
+
+ offset_fsb = XFS_B_TO_FSBT(mp, offset);
+ end_fsb = XFS_B_TO_FSB(mp, offset + count);
+
+ /*
+ * Each remapping operation could cause a btree split, so in the worst
+ * case that's one for each block.
+ */
+ resblks = (end_fsb - offset_fsb) *
+ XFS_NEXTENTADD_SPACE_RES(mp, 1, XFS_DATA_FORK);
+
+ error = xfs_trans_alloc(mp, &M_RES(mp)->tr_write, resblks, 0,
+ XFS_TRANS_RESERVE, &tp);
+ if (error)
+ return error;
+
+ xfs_ilock(ip, XFS_ILOCK_EXCL);
+ xfs_trans_ijoin(tp, ip, 0);
+
+ while (end_fsb > offset_fsb && !error) {
+ error = xfs_reflink_end_cow_extent_locked(tp, ip, &offset_fsb,
+ end_fsb);
+ }
+ if (error) {
+ trace_xfs_reflink_end_cow_error(ip, error, _RET_IP_);
+ goto out_cancel;
+ }
+ error = xfs_trans_commit(tp);
+ xfs_iunlock(ip, XFS_ILOCK_EXCL);
+ return error;
+out_cancel:
+ xfs_trans_cancel(tp);
+ xfs_iunlock(ip, XFS_ILOCK_EXCL);
+ return error;
+}
+
/*
* Free all CoW staging blocks that are still referenced by the ondisk refcount
* metadata. The ondisk metadata does not track which inode created the
diff --git a/fs/xfs/xfs_reflink.h b/fs/xfs/xfs_reflink.h
index 379619f24247..412e9b6f2082 100644
--- a/fs/xfs/xfs_reflink.h
+++ b/fs/xfs/xfs_reflink.h
@@ -45,6 +45,8 @@ extern int xfs_reflink_cancel_cow_range(struct xfs_inode *ip, xfs_off_t offset,
xfs_off_t count, bool cancel_real);
extern int xfs_reflink_end_cow(struct xfs_inode *ip, xfs_off_t offset,
xfs_off_t count);
+int xfs_reflink_end_atomic_cow(struct xfs_inode *ip, xfs_off_t offset,
+ xfs_off_t count);
extern int xfs_reflink_recover_cow(struct xfs_mount *mp);
extern loff_t xfs_reflink_remap_range(struct file *file_in, loff_t pos_in,
struct file *file_out, loff_t pos_out, loff_t len,
--
2.31.1