fs/super.c: introduce get_tree_bdev_flags()

As Allison reported [1], currently get_tree_bdev() will store
"Can't lookup blockdev" error message. Although it makes sense for
pure bdev-based fses, this message may mislead users who try to use
EROFS file-backed mounts since get_tree_nodev() is used as a fallback
then.

Add get_tree_bdev_flags() to specify extensible flags [2] and
GET_TREE_BDEV_QUIET_LOOKUP to silence "Can't lookup blockdev" message
since it's misleading to EROFS file-backed mounts now.

[1] https://lore.kernel.org/r/CAOYeF9VQ8jKVmpy5Zy9DNhO6xmWSKMB-DO8yvBB0XvBE7=[e-mail]
[2] https://lore.kernel.org/r/[e-mail]

Suggested-by: Christoph Hellwig <[e-mail]>
Signed-off-by: Gao Xiang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Reviewed-by: Christoph Hellwig <[e-mail]>
Reviewed-by: Jan Kara <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/super.c

···
 EXPORT_SYMBOL_GPL(setup_bdev_super);
 
 /**
- * get_tree_bdev - Get a superblock based on a single block device
+ * get_tree_bdev_flags - Get a superblock based on a single block device
  * @fc: The filesystem context holding the parameters
  * @fill_super: Helper to initialise a new superblock
+ * @flags: GET_TREE_BDEV_* flags
  */
-int get_tree_bdev(struct fs_context *fc,
-		int (*fill_super)(struct super_block *,
-				  struct fs_context *))
+int get_tree_bdev_flags(struct fs_context *fc,
+		int (*fill_super)(struct super_block *sb,
+				  struct fs_context *fc), unsigned int flags)
 {
 	struct super_block *s;
 	int error = 0;
···
 
 	error = lookup_bdev(fc->source, &dev);
 	if (error) {
-		errorf(fc, "%s: Can't lookup blockdev", fc->source);
+		if (!(flags & GET_TREE_BDEV_QUIET_LOOKUP))
+			errorf(fc, "%s: Can't lookup blockdev", fc->source);
 		return error;
 	}
-
 	fc->sb_flags |= SB_NOSEC;
 	s = sget_dev(fc, dev);
 	if (IS_ERR(s))
···
 	BUG_ON(fc->root);
 	fc->root = dget(s->s_root);
 	return 0;
+}
+EXPORT_SYMBOL_GPL(get_tree_bdev_flags);
+
+/**
+ * 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 *))
+{
+	return get_tree_bdev_flags(fc, fill_super, 0);
 }
 EXPORT_SYMBOL(get_tree_bdev);
 

include/linux/fs_context.h

···
 
 int setup_bdev_super(struct super_block *sb, int sb_flags,
 		struct fs_context *fc);
+
+#define GET_TREE_BDEV_QUIET_LOOKUP		0x0001
+int get_tree_bdev_flags(struct fs_context *fc,
+		int (*fill_super)(struct super_block *sb,
+				  struct fs_context *fc), unsigned int flags);
+
 extern int get_tree_bdev(struct fs_context *fc,
 			       int (*fill_super)(struct super_block *sb,
 						 struct fs_context *fc));