Add ioctl to clear unused space

This patchset adds new ioctl similar to TRIM, that provides several other ways how to clear the unused space. The changelogs are incomplete, for preview not for inclusion yet.

It compiles and has been tested lightly, the clearing modes depend on hw capabilities (secure discard, sector unmapping instead of zeros), so I've tested only zeroing and discard.

I personally think the zeroing has a usecase and the other modes were easy to add. Further extensions can be considered, eg. WRITE_SAME, overwriting with a randomly generated pattern, or some filesystem canary patterns that can be used to report unused block read as metadata.

As this is modelled after the generic FITRIM ioctl, so this could be a new generic ioctl too. However, last time somebody wanted such ioctl, there was a pushback. I'll consider making a generic version and send it for comments to fsdevel eventually.

git://github.com/kdave/btrfs-devel dev/zero-free

David Sterba (6):
btrfs: extend trim callchain to pass the operation type
btrfs: add wrapper to switch clearing operation
btrfs: add zeroing clear operation
btrfs: add new ioctl BTRFS_IOC_CLEAR_FREE
btrfs: add discard secure to clear unused space
btrfs: add more zeroout modes for clearing unused space

 fs/btrfs/ctree.h            |   5 +-
 fs/btrfs/extent-tree.c      | 130 ++++++++++++++++++++++++++++++++++++++------
 fs/btrfs/free-space-cache.c |  22 +++++---
 fs/btrfs/free-space-cache.h |   3 +-
 fs/btrfs/ioctl.c            |  42 ++++++++++++++
 include/uapi/linux/btrfs.h  |  26 +++++++++
 6 files changed, 199 insertions(+), 29 deletions(-)

2.16.2