Re: [PATCH 1/4] btrfs: add mount_root() and new file_system_type

[David Sterba <dsterba@suse.cz>]

On Mon, Sep 25, 2017 at 04:27:07PM +0900, Misono, Tomohiro wrote:
> Add mount_root() and new file_system_type for preparation of cleanup of
> btrfs_mount(). Code path is not changed yet.
> 
> mount_root() is almost the same as current btrfs_mount(), but doesn't
> have subvolume related part.
> 
> Signed-off-by: Tomohiro Misono <misono.tomohiro@jp.fujitsu.com>
> ---
>  fs/btrfs/super.c | 116 +++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 116 insertions(+)
> 
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 12540b6..fe43606 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -66,6 +66,7 @@
>  
>  static const struct super_operations btrfs_super_ops;
>  static struct file_system_type btrfs_fs_type;
> +static struct file_system_type btrfs_root_fs_type;

A comment why we need this would be useful.

>  
>  static int btrfs_remount(struct super_block *sb, int *flags, char *data);
>  
> @@ -1514,6 +1515,112 @@ static int setup_security_options(struct btrfs_fs_info *fs_info,
>  	return ret;
>  }
>  
> +static struct dentry *mount_root(struct file_system_type *fs_type, int flags,

'mount_root' is too generic and could be confused with other mount_*
functions from VFS, btrfs_mount_root would be better.

> +		const char *device_name, void *data)
> +{
> +	struct block_device *bdev = NULL;
> +	struct super_block *s;
> +	struct btrfs_fs_devices *fs_devices = NULL;
> +	struct btrfs_fs_info *fs_info = NULL;
> +	struct security_mnt_opts new_sec_opts;
> +	fmode_t mode = FMODE_READ;
> +	char *subvol_name = NULL;
> +	u64 subvol_objectid = 0;
> +	int error = 0;
> +
> +	if (!(flags & MS_RDONLY))
> +		mode |= FMODE_WRITE;
> +
> +	error = btrfs_parse_early_options(data, mode, fs_type,
> +					  &subvol_name, &subvol_objectid,
> +					  &fs_devices);
> +	if (error) {
> +		kfree(subvol_name);
> +		return ERR_PTR(error);
> +	}
> +
> +	security_init_mnt_opts(&new_sec_opts);
> +	if (data) {
> +		error = parse_security_options(data, &new_sec_opts);
> +		if (error)
> +			return ERR_PTR(error);
> +	}
> +
> +	error = btrfs_scan_one_device(device_name, mode, fs_type, &fs_devices);
> +	if (error)
> +		goto error_sec_opts;
> +
> +	/*
> +	 * Setup a dummy root and fs_info for test/set super.  This is because
> +	 * we don't actually fill this stuff out until open_ctree, but we need
> +	 * it for searching for existing supers, so this lets us do that and
> +	 * then open_ctree will properly initialize everything later.
> +	 */
> +	fs_info = kzalloc(sizeof(struct btrfs_fs_info), GFP_NOFS);

Why GFP_NOFS?

> +	if (!fs_info) {
> +		error = -ENOMEM;
> +		goto error_sec_opts;
> +	}
> +
> +	fs_info->fs_devices = fs_devices;
> +
> +	fs_info->super_copy = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_NOFS);
> +	fs_info->super_for_commit = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_NOFS);

Also here. Current code uses GFP_KERNEL so this might got switched in
the meantim.

> +	security_init_mnt_opts(&fs_info->security_opts);
> +	if (!fs_info->super_copy || !fs_info->super_for_commit) {
> +		error = -ENOMEM;
> +		goto error_fs_info;
> +	}
> +
> +	error = btrfs_open_devices(fs_devices, mode, fs_type);
> +	if (error)
> +		goto error_fs_info;
> +
> +	if (!(flags & MS_RDONLY) && fs_devices->rw_devices == 0) {
> +		error = -EACCES;
> +		goto error_close_devices;
> +	}
> +
> +	bdev = fs_devices->latest_bdev;
> +	s = sget(fs_type, btrfs_test_super, btrfs_set_super, flags | MS_NOSEC,
> +		 fs_info);
> +	if (IS_ERR(s)) {
> +		error = PTR_ERR(s);
> +		goto error_close_devices;
> +	}
> +
> +	if (s->s_root) {
> +		btrfs_close_devices(fs_devices);
> +		free_fs_info(fs_info);
> +		if ((flags ^ s->s_flags) & MS_RDONLY)
> +			error = -EBUSY;
> +	} else {
> +		snprintf(s->s_id, sizeof(s->s_id), "%pg", bdev);
> +		btrfs_sb(s)->bdev_holder = fs_type;
> +		error = btrfs_fill_super(s, fs_devices, data);
> +	}
> +	if (error) {
> +		deactivate_locked_super(s);
> +		goto error_sec_opts;
> +	}
> +
> +	fs_info = btrfs_sb(s);
> +	error = setup_security_options(fs_info, s, &new_sec_opts);
> +	if (error) {
> +		deactivate_locked_super(s);
> +		goto error_sec_opts;
> +	}
> +
> +	return dget(s->s_root);
> +
> +error_close_devices:
> +	btrfs_close_devices(fs_devices);
> +error_fs_info:
> +	free_fs_info(fs_info);
> +error_sec_opts:
> +	security_free_mnt_opts(&new_sec_opts);
> +	return ERR_PTR(error);
> +}
>  /*
>   * Find a superblock for the given device / mount point.
>   *
> @@ -2133,6 +2240,15 @@ static struct file_system_type btrfs_fs_type = {
>  	.kill_sb	= btrfs_kill_super,
>  	.fs_flags	= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA,
>  };
> +
> +static struct file_system_type btrfs_root_fs_type = {
> +	.owner		= THIS_MODULE,
> +	.name		= "btrfs",
> +	.mount		= mount_root,
> +	.kill_sb	= btrfs_kill_super,
> +	.fs_flags	= FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA,
> +};
> +
>  MODULE_ALIAS_FS("btrfs");
>  
>  static int btrfs_control_open(struct inode *inode, struct file *file)
> -- 
> 2.9.5
> 
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