[PATCH v11 13/16] xfs: add xfs_file_dio_write_atomic()

John Garry
2025-05-04T09:02:29Z

Add xfs_file_dio_write_atomic() for dedicated handling of atomic writes.

Now HW offload will not be required to support atomic writes and is
an optional feature.

CoW-based atomic writes will be supported with out-of-places write and
atomic extent remapping.

Either mode of operation may be used for an atomic write, depending on the
circumstances.

The preferred method is HW offload as it will be faster. If HW offload is
not available then we always use the CoW-based method. If HW offload is
available but not possible to use, then again we use the CoW-based method.

If available, HW offload would not be possible for the write length
exceeding the HW offload limit, the write spanning multiple extents,
unaligned disk blocks, etc.

Apart from the write exceeding the HW offload limit, other conditions for
HW offload usage can only be detected in the iomap handling for the write.
As such, we use a fallback method to issue the write if we detect in the
->iomap_begin() handler that HW offload is not possible. Special code
-ENOPROTOOPT is returned from ->iomap_begin() to inform that HW offload is
not possible.

In other words, atomic writes are supported on any filesystem that can
perform out of place write remapping atomically (i.e. reflink) up to
some fairly large size. If the conditions are right (a single correctly
aligned overwrite mapping) then the filesystem will use any available
hardware support to avoid the filesystem metadata updates.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: John Garry <john.g.garry@oracle.com>
---
fs/xfs/xfs_file.c | 68 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 68 insertions(+)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index 32883ec8ca2e..f4a66ff85748 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -728,6 +728,72 @@ xfs_file_dio_write_zoned(
return ret;
}

+/*
+ * Handle block atomic writes
+ *
+ * Two methods of atomic writes are supported:
+ * - REQ_ATOMIC-based, which would typically use some form of HW offload in the
+ * disk
+ * - COW-based, which uses a COW fork as a staging extent for data updates
+ * before atomically updating extent mappings for the range being written
+ *
+ */
+static noinline ssize_t
+xfs_file_dio_write_atomic(
+ struct xfs_inode *ip,
+ struct kiocb *iocb,
+ struct iov_iter *from)
+{
+ unsigned int iolock = XFS_IOLOCK_SHARED;
+ ssize_t ret, ocount = iov_iter_count(from);
+ const struct iomap_ops *dops;
+
+ /*
+ * HW offload should be faster, so try that first if it is already
+ * known that the write length is not too large.
+ */
+ if (ocount > xfs_inode_buftarg(ip)->bt_bdev_awu_max)
+ dops = &xfs_atomic_write_cow_iomap_ops;
+ else
+ dops = &xfs_direct_write_iomap_ops;
+
+retry:
+ ret = xfs_ilock_iocb_for_write(iocb, &iolock);
+ if (ret)
+ return ret;
+
+ ret = xfs_file_write_checks(iocb, from, &iolock, NULL);
+ if (ret)
+ goto out_unlock;
+
+ /* Demote similar to xfs_file_dio_write_aligned() */
+ if (iolock == XFS_IOLOCK_EXCL) {
+ xfs_ilock_demote(ip, XFS_IOLOCK_EXCL);
+ iolock = XFS_IOLOCK_SHARED;
+ }
+
+ trace_xfs_file_direct_write(iocb, from);
+ ret = iomap_dio_rw(iocb, from, dops, &xfs_dio_write_ops,
+ 0, NULL, 0);
+
+ /*
+ * The retry mechanism is based on the ->iomap_begin method returning
+ * -ENOPROTOOPT, which would be when the REQ_ATOMIC-based write is not
+ * possible. The REQ_ATOMIC-based method typically not be possible if
+ * the write spans multiple extents or the disk blocks are misaligned.
+ */
+ if (ret == -ENOPROTOOPT && dops == &xfs_direct_write_iomap_ops) {
+ xfs_iunlock(ip, iolock);
+ dops = &xfs_atomic_write_cow_iomap_ops;
+ goto retry;
+ }
+
+out_unlock:
+ if (iolock)
+ xfs_iunlock(ip, iolock);
+ return ret;
+}
+
/*
* Handle block unaligned direct I/O writes
*
@@ -843,6 +909,8 @@ xfs_file_dio_write(
return xfs_file_dio_write_unaligned(ip, iocb, from);
if (xfs_is_zoned_inode(ip))
return xfs_file_dio_write_zoned(ip, iocb, from);
+ if (iocb->ki_flags & IOCB_ATOMIC)
+ return xfs_file_dio_write_atomic(ip, iocb, from);
return xfs_file_dio_write_aligned(ip, iocb, from,
&xfs_direct_write_iomap_ops, &xfs_dio_write_ops, NULL);
}
--
2.31.1