Btrfs: Introduce contexts for metadata reservation

Introducing metadata reseravtion contexts has two major advantages.
First, it makes metadata reseravtion more traceable. Second, it can
reclaim freed space and re-add them to the itself after transaction
committed.

Besides add btrfs_block_rsv structure and related helper functions,
This patch contains following changes:

Move code that decides if freed tree block should be pinned into
btrfs_free_tree_block().

Make space accounting more accurate, mainly for handling read only
block groups.

Signed-off-by: Chris Mason <chris.mason@oracle.com>

1 files changed, 1 insertions, 2 deletions

diff --git a/fs/btrfs/transaction.c b/fs/btrfs/transaction.c
index 2cb116099b90..21ad37c05199 100644
--- a/fs/btrfs/transaction.c
+++ b/fs/btrfs/transaction.c
@@ -185,9 +185,8 @@ static struct btrfs_trans_handle *start_transaction(struct btrfs_root *root,
         h->blocks_reserved = num_blocks;
         h->blocks_used = 0;
         h->block_group = 0;
-        h->alloc_exclude_nr = 0;
-        h->alloc_exclude_start = 0;
         h->delayed_ref_updates = 0;
+        h->block_rsv = NULL;
 
         if (!current->journal_info && type != TRANS_USERSPACE)
                 current->journal_info = h;