Comment # 15 on bug 908706 from Neil Brown

Hi Mark,
thanks for all the details.

Part of the problem is that your /var/cache/fscache is in a 'btrfs' volume.
fscache doesn't work on btrfs.  It requires the filesystem to provide a 'bmap'
interface user e.g. by the FIBMAP ioctl.  btrfs doesn't provide that (for good
reason).
fscache (or more acturately: cachefiles which is the cache backend in use)
refuses to work if bmap isn't available, and this results in the 

[  859.668561] CacheFiles: Lookup failed error -1
[  859.672224] CacheFiles: Lookup failed error -1
[  859.675402] CacheFiles: Lookup failed error -1
[  859.694269] CacheFiles: Lookup failed error -1


messages, and results in all the files always being empty.

The crash:

[  859.700614] FS-Cache: Assertion failed
[  859.700704] ------------[ cut here ]------------
[  859.700765] kernel BUG at ../fs/fscache/operation.c:38!

seems to be caused by the object being removed from the cache before it has
even been added.  I suspect that is being caused because creating the object
fails due to the lack of 'bmap'.

Is there any way you can mount some other filesystem on /var/cache/fscache?
You could possible create a large file somewhere, then
  losetup /dev/loop0 /my/large/file
  mkfs.ext4 /dev/loop0
  mount /dev/loop0 /var/cache/fscache

Then restart cachefilesd.