[PATCH] check for O_DIRECT capability in open(), not write()

For O_DIRECT opens we're currently checking that the fs supports
O_DIRECT at write(2)-time.

This is a forward-port of Andrea's patch which moves the check to
open() time.  Seems more sensible.

fs/fcntl.c

@@ -245,6 +245,11 @@ static int setfl(int fd, struct file * filp, unsigned long arg)
 	}
 
 	if (arg & O_DIRECT) {
+		if (inode->i_mapping && inode->i_mapping->a_ops) {
+			if (!inode->i_mapping->a_ops->direct_IO)
+				return -EINVAL;
+		}
+
 		/*
 		 * alloc_kiovec() can sleep and we are only serialized by
 		 * the big kernel lock here, so abuse the i_sem to serialize

fs/open.c

@@ -665,6 +665,14 @@ struct file *dentry_open(struct dentry *dentry, struct vfsmount *mnt, int flags)
 	}
 	f->f_flags &= ~(O_CREAT | O_EXCL | O_NOCTTY | O_TRUNC);
 
+	/* NB: we're sure to have correct a_ops only after f_op->open */
+	if (f->f_flags & O_DIRECT) {
+		error = -EINVAL;
+		if (inode->i_mapping && inode->i_mapping->a_ops)
+			if (!inode->i_mapping->a_ops->direct_IO)
+				goto cleanup_all;
+	}
+
 	return f;
 
 cleanup_all:

mm/filemap.c

@@ -1121,8 +1121,6 @@ static ssize_t generic_file_direct_IO(int rw, struct file * filp, char * buf, si
 	retval = -EINVAL;
 	if ((offset & blocksize_mask) || (count & blocksize_mask))
 		goto out_free;
-	if (!mapping->a_ops->direct_IO)
-		goto out_free;
 
 	/*
 	 * Flush to disk exclusively the _data_, metadata must remain