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

[David Sterba <dsterba@suse.cz>]

On Wed, Aug 12, 2020 at 01:36:52PM -0300, Marcos Paulo de Souza wrote:
> 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.

This sounds like the adding back recursive mount with -o subvol= that
was removed.

> + */
> +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.
> +	 */

Can this race somehow? If there are several parallel mounts called eg.
for a set of system subvolumes.

> +	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