GFS2: Add atomic_open support

I've restricted atomic_open to only operate on regular files, although
I still don't understand why atomic_open should not be possible also for
directories on GFS2. That can always be added in later though, if it
makes sense.

The ->atomic_open function can be passed negative dentries, which
in most cases means either ENOENT (->lookup) or a call to d_instantiate
(->create). In the GFS2 case though, we need to actually perform the
look up, since we do not know whether there has been a new inode created
on another node. The look up calls d_splice_alias which then tries to
rehash the dentry - so the solution here is to simply check for that
in d_splice_alias. The same issue is likely to affect any other cluster
filesystem implementing ->atomic_open

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: "J. Bruce Fields" <bfields fieldses org>
Cc: Jeff Layton <jlayton@redhat.com>

fs/gfs2/file.c

@@ -537,11 +537,52 @@ static int gfs2_mmap(struct file *file, struct vm_area_struct *vma)
 	return 0;
 }
 
+/**
+ * gfs2_open_common - This is common to open and atomic_open
+ * @inode: The inode being opened
+ * @file: The file being opened
+ *
+ * This maybe called under a glock or not depending upon how it has
+ * been called. We must always be called under a glock for regular
+ * files, however. For other file types, it does not matter whether
+ * we hold the glock or not.
+ *
+ * Returns: Error code or 0 for success
+ */
+
+int gfs2_open_common(struct inode *inode, struct file *file)
+{
+	struct gfs2_file *fp;
+	int ret;
+
+	if (S_ISREG(inode->i_mode)) {
+		ret = generic_file_open(inode, file);
+		if (ret)
+			return ret;
+	}
+
+	fp = kzalloc(sizeof(struct gfs2_file), GFP_NOFS);
+	if (!fp)
+		return -ENOMEM;
+
+	mutex_init(&fp->f_fl_mutex);
+
+	gfs2_assert_warn(GFS2_SB(inode), !file->private_data);
+	file->private_data = fp;
+	return 0;
+}
+
 /**
  * gfs2_open - open a file
  * @inode: the inode to open
  * @file: the struct file for this opening
  *
+ * After atomic_open, this function is only used for opening files
+ * which are already cached. We must still get the glock for regular
+ * files to ensure that we have the file size uptodate for the large
+ * file check which is in the common code. That is only an issue for
+ * regular files though.
+ *
  * Returns: errno
  */
 
@@ -549,40 +590,22 @@ static int gfs2_open(struct inode *inode, struct file *file)
 {
 	struct gfs2_inode *ip = GFS2_I(inode);
 	struct gfs2_holder i_gh;
-	struct gfs2_file *fp;
 	int error;
-
-	fp = kzalloc(sizeof(struct gfs2_file), GFP_KERNEL);
-	if (!fp)
-		return -ENOMEM;
-
-	mutex_init(&fp->f_fl_mutex);
-
-	gfs2_assert_warn(GFS2_SB(inode), !file->private_data);
-	file->private_data = fp;
+	bool need_unlock = false;
 
 	if (S_ISREG(ip->i_inode.i_mode)) {
 		error = gfs2_glock_nq_init(ip->i_gl, LM_ST_SHARED, LM_FLAG_ANY,
 					   &i_gh);
 		if (error)
-			goto fail;
-
-		if (!(file->f_flags & O_LARGEFILE) &&
-		    i_size_read(inode) > MAX_NON_LFS) {
-			error = -EOVERFLOW;
-			goto fail_gunlock;
-		}
-
-		gfs2_glock_dq_uninit(&i_gh);
+			return error;
+		need_unlock = true;
 	}
 
-	return 0;
+	error = gfs2_open_common(inode, file);
+
+	if (need_unlock)
+		gfs2_glock_dq_uninit(&i_gh);
 
-fail_gunlock:
-	gfs2_glock_dq_uninit(&i_gh);
-fail:
-	file->private_data = NULL;
-	kfree(fp);
 	return error;
 }