fs: support RWF_NOWAIT for buffered reads

This is based on the old idea and code from Milosz Tanski. With the aio
nowait code it becomes mostly trivial now. Buffered writes continue to
return -EOPNOTSUPP if RWF_NOWAIT is passed.

Signed-off-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Signed-off-by: Al Viro <[e-mail]>

fs/aio.c

···
 		goto out_put_req;
 	}
 
-	if ((req->common.ki_flags & IOCB_NOWAIT) &&
-			!(req->common.ki_flags & IOCB_DIRECT)) {
-		ret = -EOPNOTSUPP;
-		goto out_put_req;
-	}
-
 	ret = put_user(KIOCB_KEY, &user_iocb->aio_key);
 	if (unlikely(ret)) {
 		pr_debug("EFAULT: aio_key\n");

fs/btrfs/file.c

···
 	loff_t oldsize;
 	int clean_page = 0;
 
+	if (!(iocb->ki_flags & IOCB_DIRECT) &&
+	    (iocb->ki_flags & IOCB_NOWAIT))
+		return -EOPNOTSUPP;
+
 	if (!inode_trylock(inode)) {
 		if (iocb->ki_flags & IOCB_NOWAIT)
 			return -EAGAIN;
···
 
 static int btrfs_file_open(struct inode *inode, struct file *filp)
 {
-	filp->f_mode |= FMODE_AIO_NOWAIT;
+	filp->f_mode |= FMODE_NOWAIT;
 	return generic_file_open(inode, filp);
 }
 

fs/ext4/file.c

···
 	if (IS_DAX(inode))
 		return ext4_dax_write_iter(iocb, from);
 #endif
+	if (!o_direct && (iocb->ki_flags & IOCB_NOWAIT))
+		return -EOPNOTSUPP;
 
 	if (!inode_trylock(inode)) {
 		if (iocb->ki_flags & IOCB_NOWAIT)
···
 			return ret;
 	}
 
-	/* Set the flags to support nowait AIO */
-	filp->f_mode |= FMODE_AIO_NOWAIT;
-
+	filp->f_mode |= FMODE_NOWAIT;
 	return dquot_file_open(inode, filp);
 }
 

fs/xfs/xfs_file.c

···
 
 	trace_xfs_file_buffered_read(ip, iov_iter_count(to), iocb->ki_pos);
 
-	xfs_ilock(ip, XFS_IOLOCK_SHARED);
+	if (!xfs_ilock_nowait(ip, XFS_IOLOCK_SHARED)) {
+		if (iocb->ki_flags & IOCB_NOWAIT)
+			return -EAGAIN;
+		xfs_ilock(ip, XFS_IOLOCK_SHARED);
+	}
 	ret = generic_file_read_iter(iocb, to);
 	xfs_iunlock(ip, XFS_IOLOCK_SHARED);
 
···
 	int			enospc = 0;
 	int			iolock;
 
+	if (iocb->ki_flags & IOCB_NOWAIT)
+		return -EOPNOTSUPP;
+
 write_retry:
 	iolock = XFS_IOLOCK_EXCL;
 	xfs_ilock(ip, iolock);
···
 		return -EFBIG;
 	if (XFS_FORCED_SHUTDOWN(XFS_M(inode->i_sb)))
 		return -EIO;
-	file->f_mode |= FMODE_AIO_NOWAIT;
+	file->f_mode |= FMODE_NOWAIT;
 	return 0;
 }
 

include/linux/fs.h

···
 /* File was opened by fanotify and shouldn't generate fanotify events */
 #define FMODE_NONOTIFY		((__force fmode_t)0x4000000)
 
-/* File is capable of returning -EAGAIN if AIO will block */
-#define FMODE_AIO_NOWAIT	((__force fmode_t)0x8000000)
+/* File is capable of returning -EAGAIN if I/O will block */
+#define FMODE_NOWAIT	((__force fmode_t)0x8000000)
 
 /*
  * Flag for rw_copy_check_uvector and compat_rw_copy_check_uvector
···
 		return -EOPNOTSUPP;
 
 	if (flags & RWF_NOWAIT) {
-		if (!(ki->ki_filp->f_mode & FMODE_AIO_NOWAIT))
+		if (!(ki->ki_filp->f_mode & FMODE_NOWAIT))
 			return -EOPNOTSUPP;
 		ki->ki_flags |= IOCB_NOWAIT;
 	}