Fix issue with CoW file lock

[skypodolsky]

The issue is related to these error messages noticed on PopOS (v6.0.12):

[  693.318921] elastio-snap: ioctl command received: 1074020613
[  693.318927] elastio-snap: received transition inc ioctl - 7
[  693.318929] elastio-snap: allocating device struct
[  693.318931] elastio-snap: initializing tracer
[  693.318933] elastio-snap: creating kernel cow thread
[  693.319053] elastio-snap: getting the base block device's make_request_fn
[  693.319055] elastio-snap: original mrf is already replaced with the tracing_mrf = 000000000f1649ea
[  693.319057] elastio-snap: found already traced device dm-5 with the same original bd_ops. orig mrf = 0000000007ac3a93; orig ops = 000000005947874e; tracing ops = 00000000f799ecd7
[  693.319059] elastio-snap: using already existing tracing ops for device with minor 7
[  693.319073] elastio-snap: freezing 'dm-5'
[  693.608738] elastio-snap: starting tracing
[  693.608751] elastio-snap: thawing 'dm-5'
[  693.644130] elastio-snap: stopping cow thread
[  693.644877] elastio-snap: error performing truncation: -1
[  693.644892] elastio-snap: error truncating file: -1
[  693.644906] elastio-snap: warning: failed to truncate '/tmp/elastio-snap_007/cow.snap', incremental will use more disk space than needed`
[  693.644911] elastio-snap: stopping mrf thread
[  693.644965] elastio-snap: freeing gendisk
[  693.681131] elastio-snap: freeing request queue
[  693.681591] elastio-snap: minor range = 7 - 7

It was also reproduced on Fedora 37 (v6.0.14).

The problem was caused by the usage of file_unlock() function in the incorrect place, which made the truncation impossible.

Closes #212

[e-kov]

@skypodolsky have you had a chance to verify the fix on PopOS 22.04 with the kernel 6.0.12?

[e-kov]

@skypodolsky have you had a chance to verify the fix on PopOS 22.04 with the kernel 6.0.12?

The issue is not reproduced on on PopOS 22.04 with the kernel 6.0.12 using this branch. However #213 has been reproduced. But it's a question for another investigation. Dmesg from PopOS: dmesg_popos.log