Re: [PATCH RFC 3/3] btrfs: use latest_bdev in btrfs_show_devname

[Su Yue <l@damenly.su>]

On Fri 20 Aug 2021 at 02:18, Anand Jain <anand.jain@oracle.com> 
wrote:

> latest_bdev is updated according to the changes to the device 
> list.
> That means we could use the latest_bdev to show the device name 
> in
> /proc/self/mounts. So this patch makes that change.
>
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
>
> RFC because
> 1. latest_bdev might not be the lowest devid but, we showed
> the lowest devid in /proc/self/mount.
> 2. The device's path is not shown now but, previously we did.
> So does these break ABI? Maybe yes for 2 howabout for 1 above?
>
Mabybe a naive question but I have no time to dive into btrfs code
recently. If a device which has highest devid was replaced, will 
the
new device be the fs_devices->latest_bdev instead of the existing 
older
one?

Thanks.

--
Su
>  fs/btrfs/super.c | 25 +++----------------------
>  1 file changed, 3 insertions(+), 22 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 1f9dd1a4faa3..4ad3fe174c41 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -2464,30 +2464,11 @@ static int btrfs_unfreeze(struct 
> super_block *sb)
>  static int btrfs_show_devname(struct seq_file *m, struct dentry 
>  *root)
>  {
>  	struct btrfs_fs_info *fs_info = btrfs_sb(root->d_sb);
> -	struct btrfs_device *dev, *first_dev = NULL;
> +	char name[BDEVNAME_SIZE];
>
> -	/*
> -	 * Lightweight locking of the devices. We should not need
> -	 * device_list_mutex here as we only read the device data and 
> the list
> -	 * is protected by RCU.  Even if a device is deleted during 
> the list
> -	 * traversals, we'll get valid data, the freeing callback will 
> wait at
> -	 * least until the rcu_read_unlock.
> -	 */
> -	rcu_read_lock();
> -	list_for_each_entry_rcu(dev, &fs_info->fs_devices->devices, 
> dev_list) {
> -		if (test_bit(BTRFS_DEV_STATE_MISSING, &dev->dev_state))
> -			continue;
> -		if (!dev->name)
> -			continue;
> -		if (!first_dev || dev->devid < first_dev->devid)
> -			first_dev = dev;
> -	}
> +	seq_escape(m, bdevname(fs_info->fs_devices->latest_bdev, 
> name),
> +		   " \t\n\\");
>
> -	if (first_dev)
> -		seq_escape(m, rcu_str_deref(first_dev->name), " \t\n\\");
> -	else
> -		WARN_ON(1);
> -	rcu_read_unlock();
>  	return 0;
>  }