btrfs: extent-tree-v2, gc and no meta ref counting

v1->v2:
- I decided to add the nr of expected global roots to the super block to make
  checking more straightforward, so added a patch for reading that value.
- I sent the prep patches/fixes in a different series and those were merged, so
  dropped them from this series

--- Original email ---

Hello,

This is the kernel side of the support for the GC trees and no longer tracking
metadata reference counts.

For the GC tree we're only implementing offloading the truncate to the GC tree
for now.  As new support is added we'll add code for the garbage collection for
each of the new operations.  Truncate was picked because it's simple enough to
do, gets us a nice latency win on normal workloads, and is a quick way to
validate that the GC tree is doing what it's supposed to.

This also disables the reference counting of metadata blocks.  Snapshotting and
everything reference counting related to metadata has been disabled, and will be
turned back on as the code needed to support those operations is added back.

This survives xfstests without blowing up.  Thanks,

Josef

Josef Bacik (6):
  btrfs: read the nr_global_roots from the super block
  btrfs: don't do backref modification for metadata for extent tree v2
  btrfs: add definitions and read support for the garbage collection
    tree
  btrfs: add a btrfs_first_item helper
  btrfs: turn evict_refill_and_join into a real helper
  btrfs: add garbage collection tree support

 fs/btrfs/Makefile               |   2 +-
 fs/btrfs/ctree.c                |  23 ++++
 fs/btrfs/ctree.h                |  19 ++-
 fs/btrfs/disk-io.c              |  37 ++++--
 fs/btrfs/extent-tree.c          |  13 +-
 fs/btrfs/gc-tree.c              | 223 ++++++++++++++++++++++++++++++++
 fs/btrfs/gc-tree.h              |  15 +++
 fs/btrfs/inode.c                |  65 +++-------
 fs/btrfs/print-tree.c           |   4 +
 fs/btrfs/space-info.c           |   4 +-
 fs/btrfs/transaction.c          |  52 ++++++++
 fs/btrfs/transaction.h          |   2 +
 include/uapi/linux/btrfs_tree.h |   6 +
 13 files changed, 394 insertions(+), 71 deletions(-)
 create mode 100644 fs/btrfs/gc-tree.c
 create mode 100644 fs/btrfs/gc-tree.h

-- 
2.26.3