btrfs: remove objectid from struct btrfs_inode on 64 bits platforms

On 64 bits platforms we don't really need to have a dedicated member (the
objectid field) for the inode's number since we store in the VFS inode's
i_ino member, which is an unsigned long and this type is 64 bits wide on
64 bits platforms. We only need that field in case we are on a 32 bits
platform because the unsigned long type is 32 bits wide on such platforms
See commit 33345d01522f ("Btrfs: Always use 64bit inode number") regarding
this 64/32 bits detail.

The objectid field of struct btrfs_inode is also used to store the ID of
a root for directories that are stubs for unreferenced roots. In such
cases the inode is a directory and has the BTRFS_INODE_ROOT_STUB runtime
flag set.

So in order to reduce the size of btrfs_inode structure on 64 bits
platforms we can remove the objectid member and use the VFS inode's i_ino
member instead whenever we need to get the inode number. In case the inode
is a root stub (BTRFS_INODE_ROOT_STUB set) we can use the member
last_reflink_trans to store the ID of the unreferenced root, since such
inode is a directory and reflinks can't be done against directories.

So remove the objectid fields for 64 bits platforms and alias the
last_reflink_trans field with a name of ref_root_id in a union.
On a release kernel config, this reduces the size of struct btrfs_inode
from 1040 bytes down to 1032 bytes.

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, 1 insertions, 1 deletions

diff --git a/fs/btrfs/export.c b/fs/btrfs/export.c
index 5526e25ebb3f..5da56e21ff73 100644
--- a/fs/btrfs/export.c
+++ b/fs/btrfs/export.c
@@ -40,7 +40,7 @@ static int btrfs_encode_fh(struct inode *inode, u32 *fh, int *max_len,
 	if (parent) {
 		u64 parent_root_id;
 
-		fid->parent_objectid = BTRFS_I(parent)->objectid;
+		fid->parent_objectid = btrfs_ino(BTRFS_I(parent));
 		fid->parent_gen = parent->i_generation;
 		parent_root_id = btrfs_root_id(BTRFS_I(parent)->root);