There are two pieces, the zstd_compress and zstd_decompress kernel modules, and the BtrFS patch. The patches are based off of the linux kernel master branch (version 4.10).

Zstd Kernel modules

The header is in include/linux/zstd.h.
It is split up into zstd_compress and zstd_decompress, which can be loaded independently.
Source files are in lib/zstd/.
lib/Kconfig and lib/Makefile need to be modified by applying lib/Kconfig.diff and lib/Makefile.diff respectively.
test/UserlandTest.cpp contains tests for the patch in userland by mocking the kernel headers. It can be run with the following commands:
```
cd test
make googletest
make UserlandTest
./UserlandTest
```

BtrFS

The patch is located in btrfs.diff.
Additionally fs/btrfs/zstd.c is provided as a source for convenience.
The patch seems to be working, it doesn't crash the kernel, and compresses at speeds and ratios athat are expected. It can still use some more testing for fringe features, like printing options.