[RFC PATCH 6/8] btrfs: super: Introduce btrfs_mount_root_fc

[Marcos Paulo de Souza <marcos@mpdesouza.com>]

From: Marcos Paulo de Souza <mpdesouza@suse.com>

This function will be used by the following patches to mount the root fs
before mounting a subvolume.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
---
 fs/btrfs/super.c | 41 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 41 insertions(+)

diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 6b70fb73a1ea..5bbf4b947125 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -2419,6 +2419,47 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
 	return ERR_PTR(error);
 }
 
+/*
+ * Duplicate the current fc and prepare for mounting the root.
+ * btrfs_get_tree will be called recursively, but then will check for the
+ * ctx->root being set and call btrfs_root_get_tree.
+ */
+static int btrfs_mount_root_fc(struct fs_context *fc, unsigned int rdonly)
+{
+	struct btrfs_fs_context *ctx, *root_ctx;
+	struct fs_context *root_fc;
+	struct vfsmount *root_mnt;
+	int ret;
+
+	root_fc = vfs_dup_fs_context(fc);
+	if (IS_ERR(root_fc))
+		return PTR_ERR(root_fc);
+
+	root_fc->sb_flags &= ~SB_RDONLY;
+	root_fc->sb_flags |= rdonly | SB_NOSEC;
+	root_ctx = root_fc->fs_private;
+	root_ctx->root_mnt = NULL;
+	root_ctx->root = true;
+
+	/*
+	 * fc_mount will call btrfs_get_tree again, and by checking ctx->root
+	 * being true it'll call btrfs_root_get_tree to avoid infinite recursion.
+	 */
+	root_mnt = fc_mount(root_fc);
+	if (IS_ERR(root_mnt)) {
+		ret = PTR_ERR(root_mnt);
+		goto error_fc;
+	}
+
+	ctx = fc->fs_private;
+	ctx->root_mnt = root_mnt;
+	ret = 0;
+
+error_fc:
+	put_fs_context(root_fc);
+	return ret;
+}
+
 /*
  * Mount function which is called by VFS layer.
  *
-- 
2.28.0