[PATCH 1/2] nfs: add nowait version of nfs_start_io_direct

[Dylan Yudaken <dyudaken@gmail.com>]

nfs_start_io_direct might block on existing operations to the same
inode. In order to support NOWAIT O_DIRECT reads, add a non-blocking
version of this nfs_start_io_direct that just returns -EAGAIN if locks
could not be taken.

Signed-off-by: Dylan Yudaken <dyudaken@gmail.com>
---
 fs/nfs/internal.h |  1 +
 fs/nfs/io.c       | 38 ++++++++++++++++++++++++++++++++++++--
 2 files changed, 37 insertions(+), 2 deletions(-)

diff --git a/fs/nfs/internal.h b/fs/nfs/internal.h
index 18d46b0e71dd..0c9aca624353 100644
--- a/fs/nfs/internal.h
+++ b/fs/nfs/internal.h
@@ -532,6 +532,7 @@ extern void nfs_end_io_read(struct inode *inode);
 extern  __must_check int nfs_start_io_write(struct inode *inode);
 extern void nfs_end_io_write(struct inode *inode);
 extern __must_check int nfs_start_io_direct(struct inode *inode);
+extern __must_check int nfs_start_io_direct_nowait(struct inode *inode);
 extern void nfs_end_io_direct(struct inode *inode);
 
 static inline bool nfs_file_io_is_buffered(struct nfs_inode *nfsi)
diff --git a/fs/nfs/io.c b/fs/nfs/io.c
index 8337f0ae852d..f359a19f7879 100644
--- a/fs/nfs/io.c
+++ b/fs/nfs/io.c
@@ -101,12 +101,15 @@ nfs_end_io_write(struct inode *inode)
 EXPORT_SYMBOL_GPL(nfs_end_io_write);
 
 /* Call with exclusively locked inode->i_rwsem */
-static void nfs_block_buffered(struct nfs_inode *nfsi, struct inode *inode)
+static int nfs_block_buffered(struct nfs_inode *nfsi, struct inode *inode, bool nowait)
 {
 	if (!test_bit(NFS_INO_ODIRECT, &nfsi->flags)) {
+		if (nowait && !mapping_empty(inode->i_mapping))
+			return 1;
 		set_bit(NFS_INO_ODIRECT, &nfsi->flags);
 		nfs_sync_mapping(inode->i_mapping);
 	}
+	return 0;
 }
 
 /**
@@ -143,12 +146,43 @@ nfs_start_io_direct(struct inode *inode)
 	err = down_write_killable(&inode->i_rwsem);
 	if (err)
 		return err;
-	nfs_block_buffered(nfsi, inode);
+	nfs_block_buffered(nfsi, inode, false);
 	downgrade_write(&inode->i_rwsem);
 
 	return 0;
 }
 
+/**
+ * nfs_start_io_direct_nowait - non-blocking variant of nfs_start_io_direct()
+ * @inode: file inode
+ *
+ * Try to declare that a direct I/O operation is about to start without
+ * blocking.
+ * Ensure all buffered I/O is blocked.
+ * If this could not be done without blocking then returns -EAGAIN.
+ */
+int
+nfs_start_io_direct_nowait(struct inode *inode)
+{
+	struct nfs_inode *nfsi = NFS_I(inode);
+
+	if (!down_read_trylock(&inode->i_rwsem))
+		return -EAGAIN;
+	if (test_bit(NFS_INO_ODIRECT, &nfsi->flags))
+		return 0;
+	up_read(&inode->i_rwsem);
+
+	/* Slow path: try to flip NFS_INO_ODIRECT without blocking. */
+	if (!down_write_trylock(&inode->i_rwsem))
+		return -EAGAIN;
+	if (nfs_block_buffered(nfsi, inode, true)) {
+		up_write(&inode->i_rwsem);
+		return -EAGAIN;
+	}
+	downgrade_write(&inode->i_rwsem);
+	return 0;
+}
+
 /**
  * nfs_end_io_direct - declare that the direct i/o operation is done
  * @inode: file inode
-- 
2.50.1