Description of problem:
lvextending thin-snapshot with ext.origin will result EIO errors when reading behind the end.
One may argue do not read what you did not write, but anyone attempting to write chunks smaller than 4k will get a failure (for example mkfs.ext4 does this[1])

[1] Yes I am aware it is not a good idea to format a snapshot and I should better create a new thin-volume. I tried anyway. :-)


Version-Release number of selected component (if applicable):
lvm2-2.02.100-8.el6.x86_64
lvm2-2.02.104-0.145.el6.x86_64 (Upstream lvm)
kernel-2.6.32-431.el6.x86_64

RHEL7 and F20 are all affected.

dmsetup targets:
thin-pool        v1.7.6
thin             v1.8.6
mirror           v1.12.0
striped          v1.5.6
linear           v1.1.0
error            v1.0.1


How reproducible:
100%


Steps to Reproduce:
    VG=vg
    lvcreate --thinpool pool -L 8G $VG
    lvcreate -n l1 -L 1G $VG
    lvchange -an -pr $VG/l1
    lvcreate -s -n s1 --thinpool pool $VG/l1
    lvextend -L+1G $VG/s1
    lvs
    # Reading 4k block one less and one behind the end of origin - these are both fine:
    dd if=/dev/zero of=/dev/$VG/s1 bs=4096 count=1 seek=$((1024*1024/4-1))
    dd if=/dev/zero of=/dev/$VG/s1 bs=4096 count=1 seek=$((1024*1024/4))
    # Reading 1k block one less and one behind the end of origin - the write behind end of origin fails:
    dd if=/dev/zero of=/dev/$VG/s1 bs=1024 count=1 seek=$((1024*1024-1))
    dd if=/dev/zero of=/dev/$VG/s1 bs=1024 count=1 seek=$((1024*1024)) # THIS FAILS


Actual results:
Attempt to read from extended `s1` fails with EIOs - e.g. the lvs output below says:
  /dev/vg/s1: read failed after 0 of 4096 at 2147418112: Input/output error
  /dev/vg/s1: read failed after 0 of 4096 at 2147475456: Input/output error
This seems to be not an issue (though it is reported in messages) until one does not try to write chunks smaller than 4k.

Expected results:
This should work as if the origin was zero-padded.

Additional info:

(05:10:35) [root@zaphodc1-node01:~]$ lvcreate --thinpool pool -L 8G vg
  Logical volume "lvol0" created
  Logical volume "pool" created
(05:10:55) [root@zaphodc1-node01:~]$ lvcreate -n l1 -L 1G vg
  Logical volume "l1" created
(05:11:17) [root@zaphodc1-node01:~]$ lvchange -an -pr vg/l1
  Logical volume "l1" changed.
(05:11:25) [root@zaphodc1-node01:~]$ lvcreate -s -n s1 --thinpool pool vg/l1
  Logical volume "s1" created
(05:11:54) [root@zaphodc1-node01:~]$ lvextend -L+1G vg/s1
  Extending logical volume s1 to 2.00 GiB
  Logical volume s1 successfully resized
(05:12:23) [root@zaphodc1-node01:~]$ lvs
  /dev/vg/s1: read failed after 0 of 4096 at 2147418112: Input/output error
  /dev/vg/s1: read failed after 0 of 4096 at 2147475456: Input/output error
  LV      VG       Attr       LSize   Pool Origin Data%  Move Log Cpy%Sync Convert
  lv_root VolGroup -wi-ao----   5.81g                                             
  lv_swap VolGroup -wi-ao---- 716.00m                                             
  l1      vg       ori-------   1.00g                                             
  pool    vg       twi-a-tz--   8.00g               0.00                          
  s1      vg       Vwi-a-tz--   2.00g pool l1       0.00                          

(05:12:25) [root@zaphodc1-node01:~]$ dd if=/dev/zero of=/dev/vg/s1 bs=1024 count=1 seek=$((1024*1024-1))
1+0 records in
1+0 records out
1024 bytes (1.0 kB) copied, 0.0149106 s, 68.7 kB/s

(05:12:59) [root@zaphodc1-node01:~]$ dd if=/dev/zero of=/dev/vg/s1 bs=1024 count=1 seek=$((1024*1024))
dd: writing `/dev/vg/s1': Input/output error
1+0 records in
0+0 records out
0 bytes (0 B) copied, 0.00171456 s, 0.0 kB/s

(05:13:19) [root@zaphodc1-node01:~]$ dd if=/dev/zero of=/dev/vg/s1 bs=4096 count=1 seek=$((1024*1024/4-1))
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.00187546 s, 2.2 MB/s

(05:13:33) [root@zaphodc1-node01:~]$ dd if=/dev/zero of=/dev/vg/s1 bs=4096 count=1 seek=$((1024*1024/4))
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.0215167 s, 190 kB/s