vfs: Create fs_context-aware mount_bdev() replacement

Create a function, get_tree_bdev(), that is fs_context-aware and a
->get_tree() counterpart of mount_bdev().

It caches the block device pointer in the fs_context struct so that this
information can be passed into sget_fc()'s test and set functions.

Signed-off-by: David Howells <[e-mail]>
cc: Jens Axboe <[e-mail]>
cc: [e-mail]
Signed-off-by: Al Viro <[e-mail]>

fs/super.c

···
 EXPORT_SYMBOL(get_tree_keyed);
 
 #ifdef CONFIG_BLOCK
+
 static int set_bdev_super(struct super_block *s, void *data)
 {
 	s->s_bdev = data;
···
 
 	return 0;
 }
+
+static int set_bdev_super_fc(struct super_block *s, struct fs_context *fc)
+{
+	return set_bdev_super(s, fc->sget_key);
+}
+
+static int test_bdev_super_fc(struct super_block *s, struct fs_context *fc)
+{
+	return s->s_bdev == fc->sget_key;
+}
+
+/**
+ * get_tree_bdev - Get a superblock based on a single block device
+ * @fc: The filesystem context holding the parameters
+ * @fill_super: Helper to initialise a new superblock
+ */
+int get_tree_bdev(struct fs_context *fc,
+		int (*fill_super)(struct super_block *,
+				  struct fs_context *))
+{
+	struct block_device *bdev;
+	struct super_block *s;
+	fmode_t mode = FMODE_READ | FMODE_EXCL;
+	int error = 0;
+
+	if (!(fc->sb_flags & SB_RDONLY))
+		mode |= FMODE_WRITE;
+
+	if (!fc->source)
+		return invalf(fc, "No source specified");
+
+	bdev = blkdev_get_by_path(fc->source, mode, fc->fs_type);
+	if (IS_ERR(bdev)) {
+		errorf(fc, "%s: Can't open blockdev", fc->source);
+		return PTR_ERR(bdev);
+	}
+
+	/* Once the superblock is inserted into the list by sget_fc(), s_umount
+	 * will protect the lockfs code from trying to start a snapshot while
+	 * we are mounting
+	 */
+	mutex_lock(&bdev->bd_fsfreeze_mutex);
+	if (bdev->bd_fsfreeze_count > 0) {
+		mutex_unlock(&bdev->bd_fsfreeze_mutex);
+		warnf(fc, "%pg: Can't mount, blockdev is frozen", bdev);
+		return -EBUSY;
+	}
+
+	fc->sb_flags |= SB_NOSEC;
+	fc->sget_key = bdev;
+	s = sget_fc(fc, test_bdev_super_fc, set_bdev_super_fc);
+	mutex_unlock(&bdev->bd_fsfreeze_mutex);
+	if (IS_ERR(s))
+		return PTR_ERR(s);
+
+	if (s->s_root) {
+		/* Don't summarily change the RO/RW state. */
+		if ((fc->sb_flags ^ s->s_flags) & SB_RDONLY) {
+			warnf(fc, "%pg: Can't mount, would change RO state", bdev);
+			deactivate_locked_super(s);
+			blkdev_put(bdev, mode);
+			return -EBUSY;
+		}
+
+		/*
+		 * s_umount nests inside bd_mutex during
+		 * __invalidate_device().  blkdev_put() acquires
+		 * bd_mutex and can't be called under s_umount.  Drop
+		 * s_umount temporarily.  This is safe as we're
+		 * holding an active reference.
+		 */
+		up_write(&s->s_umount);
+		blkdev_put(bdev, mode);
+		down_write(&s->s_umount);
+	} else {
+		s->s_mode = mode;
+		snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
+		sb_set_blocksize(s, block_size(bdev));
+		error = fill_super(s, fc);
+		if (error) {
+			deactivate_locked_super(s);
+			return error;
+		}
+
+		s->s_flags |= SB_ACTIVE;
+		bdev->bd_super = s;
+	}
+
+	BUG_ON(fc->root);
+	fc->root = dget(s->s_root);
+	return 0;
+}
+EXPORT_SYMBOL(get_tree_bdev);
 
 static int test_bdev_super(struct super_block *s, void *data)
 {

include/linux/fs_context.h

···
 	struct mutex		uapi_mutex;	/* Userspace access mutex */
 	struct file_system_type	*fs_type;
 	void			*fs_private;	/* The filesystem's context */
+	void			*sget_key;
 	struct dentry		*root;		/* The root and superblock */
 	struct user_namespace	*user_ns;	/* The user namespace for this mount */
 	struct net		*net_ns;	/* The network namespace for this mount */
···
 			 int (*fill_super)(struct super_block *sb,
 					   struct fs_context *fc),
 			 void *key);
+
+extern int get_tree_bdev(struct fs_context *fc,
+			       int (*fill_super)(struct super_block *sb,
+						 struct fs_context *fc));
 
 extern const struct file_operations fscontext_fops;