virstor: write large maps in chunks during label

[rlibby]

Summary

During the initial label of a virstor device, write out the allocation
map in chunks if it is large (> 1 MB) in order to avoid large mallocs.

Even though the kernel virstor geom may still do a large malloc to
represent the allocation map, this may still be useful to avoid a
ulimit.

Sponsored by: Dell EMC Isilon

---

I am unsure if geom virstor is generally in use these days. My web
searches have not turned up much. I am interested in using it to
simulate very large drives for test purposes. In my scenario, I may
need a very large map (gigabytes). So I am looking at making virstor
changes to support this.

Test Plan

New test in D45535

% sudo kyua debug -k /usr/tests/sys/Kyuafile geom/class/virstor/virstor_test:basic
Executing command [ gvirstor label -v -s 64M -m 512 virstor.bjyhzA /dev/md0 ]
Executing command [ gvirstor destroy virstor.bjyhzA ]

Destroying test virstor device: virstor.bjyhzA
Removing test memory disk: md0
geom/class/virstor/virstor_test:basic -> passed

[markj]

lib/geom/virstor/geom_virstor.c

344–346

Not your bug, but, shouldn't we return here?

366

Do you need to free the chunk in this error path?

[rlibby]

lib/geom/virstor/geom_virstor.c

344–346

Yes... I guess we EFAULT on the write and exit then. I'll fix it.

Looking around, there are other instances of that too. The fd check just above I can also sneak into this. For the "Device %s is too small" case, I'm not sure if that was intended to be a hard error or a warning, so I'll leave that alone for the moment.

366

I guess I should. We're about to exit, but it could avoid tripping static analysis or a memory checker.

[markj]

lib/geom/virstor/geom_virstor.c

366

Since this is library code, I'd tend to be more careful about cleaning up in cases like this.

[rlibby]

markj feedback: clean up error paths