The first mount of a non-trivial file system after a btrfs_convert, or
an ongoing btrfs balance operation containing large files may lead to an
oops (and a pathologically damaged file system) if the hang check timer
(CONFIG_DETECT_HUNG_TASK=y) is compiled into the linux kernel and not
disabled.
I've had two systems destroyed after a btrfs_convert. After the
conversion the first mount took several minutes. The hung task timer
expired against some internal btrfs_daemon. I think it was
'[btrfs-transacti]'. Said task then goes oops and the file system was
chock full of errors. So many that I no longer trusted the conversion so
mkfs.btrfs and restored from backup.
On another system the same thing happened after a successful convert and
mount (I'd remembered to disable the timer during the first mount) when
a btrfs balance was running.
Whatever is blocking in that task really ought not to do that for 2+
minutes and sleep on some data structure instead.
As it is, the two options are not happy together. Be sure to
echo 0 > /proc/sys/kernel/hung_task_timeout_secs
to disable the timer before doing a mount or balance after a
btrfs_convert (and possibly a btrfs balance if it decides to move a very
large file like a VM disk image).
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