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, 2 deletions

diff --git a/fs/btrfs/tests/btrfs-tests.c b/fs/btrfs/tests/btrfs-tests.c
index b28a79935d8e..ce50847e1e01 100644
--- a/fs/btrfs/tests/btrfs-tests.c
+++ b/fs/btrfs/tests/btrfs-tests.c
@@ -61,8 +61,7 @@ struct inode *btrfs_new_test_inode(void)
 		return NULL;
 
 	inode->i_mode = S_IFREG;
-	inode->i_ino = BTRFS_FIRST_FREE_OBJECTID;
-	BTRFS_I(inode)->objectid = BTRFS_FIRST_FREE_OBJECTID;
+	btrfs_set_inode_number(BTRFS_I(inode), BTRFS_FIRST_FREE_OBJECTID);
 	inode_init_owner(&nop_mnt_idmap, inode, NULL, S_IFREG);
 
 	return inode;