btrfs: unify index_cnt and csum_bytes from struct btrfs_inode

The index_cnt field of struct btrfs_inode is used only for two purposes:

1) To store the index for the next entry added to a directory;

2) For the data relocation inode to track the logical start address of the
   block group currently being relocated.

For the relocation case we use index_cnt because it's not used for
anything else in the relocation use case - we could have used other fields
that are not used by relocation such as defrag_bytes, last_unlink_trans
or last_reflink_trans for example (among others).

Since the csum_bytes field is not used for directories, do the following
changes:

1) Put index_cnt and csum_bytes in a union, and index_cnt is only
   initialized when the inode is a directory. The csum_bytes is only
   accessed in IO paths for regular files, so we're fine here;

2) Use the defrag_bytes field for relocation, since the data relocation
   inode is never used for defrag purposes. And to make the naming better,
   alias it to reloc_block_group_start by using a union.

This reduces the size of struct btrfs_inode by 8 bytes in a release
kernel, from 1056 bytes down to 1048 bytes.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/fs/btrfs/tree-log.c b/fs/btrfs/tree-log.c
index 0bce1d45e252..e6294d2bb78b 100644
--- a/fs/btrfs/tree-log.c
+++ b/fs/btrfs/tree-log.c
@@ -1644,7 +1644,8 @@ static noinline int fixup_inode_link_count(struct btrfs_trans_handle *trans,
 		if (ret)
 			goto out;
 	}
-	BTRFS_I(inode)->index_cnt = (u64)-1;
+	if (S_ISDIR(inode->i_mode))
+		BTRFS_I(inode)->index_cnt = (u64)-1;
 
 	if (inode->i_nlink == 0) {
 		if (S_ISDIR(inode->i_mode)) {