[PATCH RFC 07/10] xfs: Add xfs_file_dio_write_atomic()

[John Garry <john.g.garry@oracle.com>]

Add xfs_file_dio_write_atomic() for dedicated handling of atomic writes.

In case of -EAGAIN being returned from iomap_dio_rw(), reissue the write
in CoW-based atomic write mode.

In the CoW-based atomic write mode, first unshare blocks so that we don't
have a cow fork for the data in the range which we are writing.

Signed-off-by: John Garry <john.g.garry@oracle.com>
---
 fs/xfs/xfs_file.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/fs/xfs/xfs_file.c b/fs/xfs/xfs_file.c
index fd05b66aea3f..12af5cdc3094 100644
--- a/fs/xfs/xfs_file.c
+++ b/fs/xfs/xfs_file.c
@@ -619,6 +619,55 @@ xfs_file_dio_write_aligned(
 	return ret;
 }
 
+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;
+	bool			use_cow = false;
+	unsigned int		dio_flags;
+	ssize_t			ret;
+
+retry:
+	ret = xfs_ilock_iocb_for_write(iocb, &iolock);
+	if (ret)
+		return ret;
+
+	ret = xfs_file_write_checks(iocb, from, &iolock);
+	if (ret)
+		goto out_unlock;
+
+	if (use_cow) {
+		ret = xfs_reflink_unshare(ip, iocb->ki_pos,
+			iov_iter_count(from));
+		if (ret)
+			goto out_unlock;
+	}
+
+	trace_xfs_file_direct_write(iocb, from);
+	if (use_cow)
+		dio_flags = IOMAP_DIO_ATOMIC_COW;
+	else
+		dio_flags = 0;
+
+	ret = iomap_dio_rw(iocb, from, &xfs_direct_write_iomap_ops,
+			&xfs_dio_write_ops, dio_flags, NULL, 0);
+
+	if (ret == -EAGAIN && !(iocb->ki_flags & IOCB_NOWAIT) && !use_cow) {
+		xfs_iunlock(ip, iolock);
+		iolock = XFS_IOLOCK_EXCL;
+		use_cow = true;
+		goto retry;
+	}
+
+out_unlock:
+	if (iolock)
+		xfs_iunlock(ip, iolock);
+	return ret;
+}
+
 /*
  * Handle block unaligned direct I/O writes
  *
@@ -723,6 +772,8 @@ xfs_file_dio_write(
 		return -EINVAL;
 	if ((iocb->ki_pos | count) & ip->i_mount->m_blockmask)
 		return xfs_file_dio_write_unaligned(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);
 }
 
-- 
2.31.1