NFS O_DIRECT reads already (mostly) handle async requests, with the
exception of locking the inode for direct.
Handle async requests properly by using nfs_start_io_direct_nowait,
and then expose FMODE_NOWAIT since it's now supported for direct reads.

Signed-off-by: Dylan Yudaken <dyudaken@gmail.com>
---
 fs/nfs/direct.c | 12 ++++++++++--
 fs/nfs/file.c   | 16 +++++++++++++++-
 2 files changed, 25 insertions(+), 3 deletions(-)

diff --git a/fs/nfs/direct.c b/fs/nfs/direct.c
index 48d89716193a..e626c72495e6 100644
--- a/fs/nfs/direct.c
+++ b/fs/nfs/direct.c
@@ -466,14 +466,22 @@ ssize_t nfs_file_direct_read(struct kiocb *iocb, struct iov_iter *iter,
 		goto out_release;
 	}
 	dreq->l_ctx = l_ctx;
-	if (!is_sync_kiocb(iocb))
+	if (!is_sync_kiocb(iocb)) {
 		dreq->iocb = iocb;
+	} else if (iocb->ki_flags & IOCB_NOWAIT) {
+		result = -EAGAIN;
+		nfs_direct_req_release(dreq);
+		goto out_release;
+	}
 
 	if (user_backed_iter(iter))
 		dreq->flags = NFS_ODIRECT_SHOULD_DIRTY;
 
 	if (!swap) {
-		result = nfs_start_io_direct(inode);
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			result = nfs_start_io_direct_nowait(inode);
+		else
+			result = nfs_start_io_direct(inode);
 		if (result) {
 			/* release the reference that would usually be
 			 * consumed by nfs_direct_read_schedule_iovec()
diff --git a/fs/nfs/file.c b/fs/nfs/file.c
index 25048a3c2364..a0d8f1c1cf10 100644
--- a/fs/nfs/file.c
+++ b/fs/nfs/file.c
@@ -72,8 +72,12 @@ nfs_file_open(struct inode *inode, struct file *filp)
 		return res;
 
 	res = nfs_open(inode, filp);
-	if (res == 0)
+	if (res == 0) {
 		filp->f_mode |= FMODE_CAN_ODIRECT;
+		/* flag NOWAIT on read-only files only */
+		if (!(filp->f_mode & FMODE_WRITE))
+			filp->f_mode |= FMODE_NOWAIT;
+	}
 	return res;
 }
 
@@ -166,6 +170,10 @@ nfs_file_read(struct kiocb *iocb, struct iov_iter *to)
 	if (iocb->ki_flags & IOCB_DIRECT)
 		return nfs_file_direct_read(iocb, to, false);
 
+	/* NOWAIT only supported on direct reads */
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EAGAIN;
+
 	dprintk("NFS: read(%pD2, %zu@%lu)\n",
 		iocb->ki_filp,
 		iov_iter_count(to), (unsigned long) iocb->ki_pos);
@@ -705,6 +713,12 @@ ssize_t nfs_file_write(struct kiocb *iocb, struct iov_iter *from)
 
 	trace_nfs_file_write(iocb, from);
 
+	/*
+	 * FMODE_NOWAIT is not set for writable files
+	 */
+	if (WARN_ON_ONCE(iocb->ki_flags & IOCB_NOWAIT))
+		return -EAGAIN;
+
 	result = nfs_key_timeout_notify(file, inode);
 	if (result)
 		return result;
-- 
2.50.1