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, 7 insertions, 10 deletions

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index c460322e3aa3..655d63748545 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4340,7 +4340,7 @@ static int btrfs_unlink_subvol(struct btrfs_trans_handle *trans,
 	if (btrfs_ino(inode) == BTRFS_FIRST_FREE_OBJECTID) {
 		objectid = btrfs_root_id(inode->root);
 	} else if (btrfs_ino(inode) == BTRFS_EMPTY_SUBVOL_DIR_OBJECTID) {
-		objectid = inode->objectid;
+		objectid = inode->ref_root_id;
 	} else {
 		WARN_ON(1);
 		fscrypt_free_filename(&fname);
@@ -5581,8 +5581,7 @@ static int btrfs_init_locked_inode(struct inode *inode, void *p)
 {
 	struct btrfs_iget_args *args = p;
 
-	inode->i_ino = args->ino;
-	BTRFS_I(inode)->objectid = args->ino;
+	btrfs_set_inode_number(BTRFS_I(inode), args->ino);
 	BTRFS_I(inode)->root = btrfs_grab_root(args->root);
 
 	if (args->root && args->root == args->root->fs_info->tree_root &&
@@ -5596,7 +5595,7 @@ static int btrfs_find_actor(struct inode *inode, void *opaque)
 {
 	struct btrfs_iget_args *args = opaque;
 
-	return args->ino == BTRFS_I(inode)->objectid &&
+	return args->ino == btrfs_ino(BTRFS_I(inode)) &&
 		args->root == BTRFS_I(inode)->root;
 }
 
@@ -5673,11 +5672,11 @@ static struct inode *new_simple_dir(struct inode *dir,
 		return ERR_PTR(-ENOMEM);
 
 	BTRFS_I(inode)->root = btrfs_grab_root(root);
-	BTRFS_I(inode)->objectid = key->objectid;
+	BTRFS_I(inode)->ref_root_id = key->objectid;
 	set_bit(BTRFS_INODE_ROOT_STUB, &BTRFS_I(inode)->runtime_flags);
 	set_bit(BTRFS_INODE_DUMMY, &BTRFS_I(inode)->runtime_flags);
 
-	inode->i_ino = BTRFS_EMPTY_SUBVOL_DIR_OBJECTID;
+	btrfs_set_inode_number(BTRFS_I(inode), BTRFS_EMPTY_SUBVOL_DIR_OBJECTID);
 	/*
 	 * We only need lookup, the rest is read-only and there's no inode
 	 * associated with the dentry
@@ -6150,7 +6149,7 @@ static int btrfs_insert_inode_locked(struct inode *inode)
 {
 	struct btrfs_iget_args args;
 
-	args.ino = BTRFS_I(inode)->objectid;
+	args.ino = btrfs_ino(BTRFS_I(inode));
 	args.root = BTRFS_I(inode)->root;
 
 	return insert_inode_locked4(inode,
@@ -6282,7 +6281,7 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 	ret = btrfs_get_free_objectid(root, &objectid);
 	if (ret)
 		goto out;
-	inode->i_ino = objectid;
+	btrfs_set_inode_number(BTRFS_I(inode), objectid);
 
 	ret = xa_reserve(&root->inodes, objectid, GFP_NOFS);
 	if (ret)
@@ -6332,8 +6331,6 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 				BTRFS_INODE_NODATASUM;
 	}
 
-	BTRFS_I(inode)->objectid = objectid;
-
 	ret = btrfs_insert_inode_locked(inode);
 	if (ret < 0) {
 		if (!args->orphan)