Btrfs: transaction ioctls

These ioctls let a user application hold a transaction open while it
performs a series of operations.  A final ioctl does a sync on the fs
(closing the current transaction).  This is the main requirement for
Ceph's OSD to be able to keep the data it's storing in a btrfs volume
consistent, and AFAICS it works just fine.  The application would do
something like

	fd = ::open("some/file", O_RDONLY);
	::ioctl(fd, BTRFS_IOC_TRANS_START);
	/* do a bunch of stuff */
	::ioctl(fd, BTRFS_IOC_TRANS_END);
or just
	::close(fd);

And to ensure it commits to disk,

	::ioctl(fd, BTRFS_IOC_SYNC);

When a transaction is held open, the trans_handle is attached to the
struct file (via private_data) so that it will get cleaned up if the
process dies unexpectedly.  A held transaction is also ended on fsync() to
avoid a deadlock.

A misbehaving application could also deliberately hold a transaction open,
effectively locking up the FS, so it may make sense to restrict something
like this to root or something.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/file.c

@@ -978,9 +978,11 @@ out_nolock:
 	return num_written ? num_written : err;
 }
 
-static int btrfs_release_file (struct inode * inode, struct file * filp)
+int btrfs_release_file(struct inode * inode, struct file * filp)
 {
 	btrfs_del_ordered_inode(inode);
+	if (filp->private_data)
+		btrfs_ioctl_trans_end(filp);
 	return 0;
 }
 
@@ -1011,6 +1013,9 @@ static int btrfs_sync_file(struct file *file,
 	/*
 	 * ok we haven't committed the transaction yet, lets do a commit
 	 */
+	if (file->private_data)
+		btrfs_ioctl_trans_end(file);
+
 	trans = btrfs_start_transaction(root, 1);
 	if (!trans) {
 		ret = -ENOMEM;