Re: [PATCH] btrfs: handle the BUG_ON in btrfs_num_devices()

[Nikolay Borisov <nborisov@suse.com>]

On  3.08.2018 15:45, Anand Jain wrote:
> Its a logical bug if we hit fs_devices::num_devices == 1 and if the
> replace is running because, as fs_devices::num_devices counts the in memory
> devices, so it should include the replace target which is running as
> indicated by the flag. If this happens return the -EINVAL back.
> 
> Suggested-by: Nikolay Borisov <nborisov@suse.com>
> Signed-off-by: Anand Jain <anand.jain@oracle.com>
> ---
> Hi,
>  As it fixes the BUG_ON I have spun a new patch for this.
>  Instead of -EINVAL should we use ASSERT?
> 
>  fs/btrfs/volumes.c | 27 ++++++++++++++++++++-------
>  1 file changed, 20 insertions(+), 7 deletions(-)
> 
> diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> index 7359596ac8eb..ed2399caff80 100644
> --- a/fs/btrfs/volumes.c
> +++ b/fs/btrfs/volumes.c
> @@ -1855,9 +1855,11 @@ void btrfs_assign_next_active_device(struct btrfs_device *device,
>  }
>  
>  /* Returns btrfs_fs_devices::num_devices minus replace device if any */
> -static u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
> +static int btrfs_num_devices(struct btrfs_fs_info *fs_info, u64 *num_devices)

Why do you resort to this travesty of returning the value in an input
parameter? Having the function return int, assuming that we will always
have a positive device num and in case of an error return a negative
value. In the worst case when we get to see a btrfs fs consisting of 2
billion devices then we can start worrying that an int here won't do it.

>  {
> -	u64 num_devices = fs_info->fs_devices->num_devices;
> +	int ret = 0;
> +
> +	*num_devices = fs_info->fs_devices->num_devices;
>  
>  	/*
>  	 * balance and replace co-exists in a scenario as below..
> @@ -1867,12 +1869,13 @@ static u64 btrfs_num_devices(struct btrfs_fs_info *fs_info)
>  	 */
>  	btrfs_dev_replace_read_lock(&fs_info->dev_replace);
>  	if (btrfs_dev_replace_is_ongoing(&fs_info->dev_replace)) {
> -		BUG_ON(num_devices < 1);
> -		num_devices--;
> +		if (*num_devices < 1)
> +			ret = -EINVAL;
> +		(*num_devices)--;
>  	}
>  	btrfs_dev_replace_read_unlock(&fs_info->dev_replace);
>  
> -	return num_devices;
> +	return ret;
>  }
>  
>  int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
> @@ -1886,7 +1889,12 @@ int btrfs_rm_device(struct btrfs_fs_info *fs_info, const char *device_path,
>  
>  	mutex_lock(&uuid_mutex);
>  
> -	num_devices = btrfs_num_devices(fs_info);
> +	ret = btrfs_num_devices(fs_info, &num_devices);
> +	if (ret) {

The canonical form, used across the whole code base of btrfs, for
checking for an error is 'if (ret <0)' as such please stick to it in
this and all future patches.

(I have a vague recollection this is not the first time I have given you
this feedback)

> +		btrfs_err(fs_info, "logical bug num_devices %llu < 0",
> +			  num_devices);
> +		return ret;
> +	}
>  
>  	ret = btrfs_check_raid_min_devices(fs_info, num_devices - 1);
>  	if (ret)
> @@ -3755,7 +3763,12 @@ int btrfs_balance(struct btrfs_fs_info *fs_info,
>  		}
>  	}
>  
> -	num_devices = btrfs_num_devices(fs_info);
> +	ret = btrfs_num_devices(fs_info, &num_devices);
> +	if (ret) {
ditto
> +		btrfs_err(fs_info, "hits a logical bug num_devices %llu < 0",
> +			  num_devices);
> +		return ret;
> +	}
>  
>  	allowed = BTRFS_AVAIL_ALLOC_BIT_SINGLE | BTRFS_BLOCK_GROUP_DUP;
>  	if (num_devices > 1)
>