[linux-lvm] Suggestion: lvm2 filters lib needs to show some readable info for end user

[heming.zhao at suse.com]

I just built & ran tag 2.3.10 code, the behavior like before.
(looks it should output: "device has a signature")
```
[tb-clustermd1 ~]# dd if=/dev/zero of=/dev/vdb bs=1M
dd: error writing '/dev/vdb': No space left on device
101+0 records in
100+0 records out
104857600 bytes (105 MB, 100 MiB) copied, 0.0600205 s, 1.7 GB/s
[tb-clustermd1 ~]# dd if=/dev/zero of=/dev/vdc bs=1M
dd: error writing '/dev/vdc': No space left on device
101+0 records in
100+0 records out
104857600 bytes (105 MB, 100 MiB) copied, 0.0566928 s, 1.8 GB/s
[tb-clustermd1 ~]# lsblk
NAME   MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
vda    253:0    0   40G  0 disk 
├─vda1 253:1    0    8M  0 part 
├─vda2 253:2    0   38G  0 part /
└─vda3 253:3    0    2G  0 part [SWAP]
vdb    253:16   0  100M  0 disk 
vdc    253:32   0  100M  0 disk 
[tb-clustermd1 ~]# parted -s /dev/vdb mklabel gpt
[tb-clustermd1 ~]# parted -s /dev/vdb mkpart parimary 0% 100%
Warning: The resulting partition is not properly aligned for best performance: 34s % 2048s != 0s
[tb-clustermd1 ~]# mkfs.ext4 /dev/vdb1 
mke2fs 1.45.6 (20-Mar-2020)
Creating filesystem with 102364 1k blocks and 25688 inodes
Filesystem UUID: 56f60edd-a235-460b-b868-3c3d1b5e4e01
Superblock backups stored on blocks: 
        8193, 24577, 40961, 57345, 73729

Allocating group tables: done                            
Writing inode tables: done                            
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done 

[tb-clustermd1 ~]# parted -s /dev/vdb rm 1
[tb-clustermd1 ~]# vgcreate testvg /dev/vdc /dev/vdb
  Device /dev/vdb excluded by a filter.
```


btw, I just built & ran latest master branch code, I saw lots of logs, it looks the code buggy/unstable.
```
# ./tools/lvm vgcreate testvg /dev/vdc /dev/vdb
  Device open /dev/sda already open with di -8193 fd -1
  Device open /dev/vda already open with di -8193 fd -1
  Device open /dev/vda1 already open with di -8193 fd -1
  Device open /dev/vda2 already open with di -8193 fd -1
  Device open /dev/vda3 already open with di -8193 fd -1
  Device open /dev/sdb already open with di -8193 fd -1
  Device open /dev/vdb already open with di -8193 fd -1
  Device open /dev/sdc already open with di -8193 fd -1
  Device open /dev/vdc already open with di -8193 fd -1
  Device open /dev/sdd already open with di -8193 fd -1
  Device open /dev/sde already open with di -8193 fd -1
  Device open /dev/sdf already open with di -8193 fd -1
  Device open /dev/sda already open with di -8193 fd -1
  Device open /dev/vda already open with di -8193 fd -1
  Device open /dev/vda1 already open with di -8193 fd -1
  Device open /dev/vda2 already open with di -8193 fd -1
  Device open /dev/vda3 already open with di -8193 fd -1
  Device open /dev/sdb already open with di -8193 fd -1
  Device open /dev/vdb already open with di -8193 fd -1
  Device open /dev/sdc already open with di -8193 fd -1
  Device open /dev/vdc already open with di -8193 fd -1
  Device open /dev/sdd already open with di -8193 fd -1
  Device open /dev/sde already open with di -8193 fd -1
  Device open /dev/sdf already open with di -8193 fd -1
  Device open /dev/vdc already open with di -8193 fd -1
  Device open /dev/vdc already open with di -8193 fd -1
  Device open /dev/vdb already open with di -8193 fd -1
  Device open /dev/vdb already open with di -8193 fd -1
  Device open /dev/vdc already open with di -8193 fd -1
  Device open /dev/vdb already open with di -8193 fd -1
  Device open /dev/vdc already open with di -8193 fd -1
  Device open /dev/vdb already open with di -8193 fd -1
```

On 10/15/20 11:16 PM, David Teigland wrote:
> On Thu, Oct 15, 2020 at 11:05:38PM +0800, heming.zhao at suse.com wrote:
>> Hello David,
>>
>> I don't know why I could use "git show 1fa48ba9d60ad34c5957b80024ef660ffb11161f" to
>> see your patch, but I can't find the commit in "git log" output (in master branch).
>> And the related code also doesn't be changed in master branch.
> 
> See these commits in the master branch:
> 450f272b31ef95bd7cb61812d9ef0525c7c1c341
> 74ed6e8a9969bb7d20c21f88b53e6134d81eab1d
> 0143c7aebe2389ceada6ca0a314a3ff0fd9cdfef
> 
> (The link I sent previously wasn't merged.)
>