Hello,

tested with linux-2.6.git a4f1cb9f3c42e5d278b0678ed5225d9331a8dcff

If i have spare disk i can grow
raid1, raid4, raid5 and raid6

but can't convert to other raid level. Raid conversation matrix:

from\to | 0 | 1 | 4 | 5 | 6
============================
  raid0  | -   -   -   -   -
  raid1  | -   +   -   -   -
  raid4  |-22 -22  +  -22 -22
  raid5  |-22 -22 -22  +  -22
  raid6  |-22 -22 -22 -22  +

Sample test script, log, dmesg, kernel config is attached.

Major problem is than can not do online covert from raid5 to raid6:

## create raid5
# mdadm --create /dev/md0 --level=5 --raid-devices=2 \
  /dev/loop1 /dev/loop2
mdadm: array /dev/md0 started.

## add some spare devices
# mdadm --add /dev/md0 /dev/loop5 /dev/loop6
mdadm: added /dev/loop5
mdadm: added /dev/loop6

## try to convert to raid6
# mdadm --grow /dev/md0 --level=6 --raid-devices=4
mdadm: Need to backup 128K of critical section..
mdadm: Cannot set device size/shape for /dev/md0: Invalid argument
dmesg: md: couldn't update array info. -22