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 33345d01 ("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>

fs/btrfs/btrfs_inode.h

@@ -129,15 +129,14 @@ struct btrfs_inode {
 	/* which subvolume this inode belongs to */
 	struct btrfs_root *root;
 
+#if BITS_PER_LONG == 32
 	/*
-	 * This is either:
-	 *
-	 * 1) The objectid of the corresponding BTRFS_INODE_ITEM_KEY;
-	 *
-	 * 2) In case this a root stub inode (BTRFS_INODE_ROOT_STUB flag set),
-	 *    the ID of that root.
+	 * The objectid of the corresponding BTRFS_INODE_ITEM_KEY.
+	 * On 64 bits platforms we can get it from vfs_inode.i_ino, which is an
+	 * unsigned long and therefore 64 bits on such platforms.
 	 */
 	u64 objectid;
+#endif
 
 	/* Cached value of inode property 'compression'. */
 	u8 prop_compress;
@@ -301,16 +300,25 @@ struct btrfs_inode {
 	 */
 	u64 last_unlink_trans;
 
-	/*
-	 * The id/generation of the last transaction where this inode was
-	 * either the source or the destination of a clone/dedupe operation.
-	 * Used when logging an inode to know if there are shared extents that
-	 * need special care when logging checksum items, to avoid duplicate
-	 * checksum items in a log (which can lead to a corruption where we end
-	 * up with missing checksum ranges after log replay).
-	 * Protected by the vfs inode lock.
-	 */
-	u64 last_reflink_trans;
+	union {
+		/*
+		 * The id/generation of the last transaction where this inode
+		 * was either the source or the destination of a clone/dedupe
+		 * operation. Used when logging an inode to know if there are
+		 * shared extents that need special care when logging checksum
+		 * items, to avoid duplicate checksum items in a log (which can
+		 * lead to a corruption where we end up with missing checksum
+		 * ranges after log replay). Protected by the VFS inode lock.
+		 * Used for regular files only.
+		 */
+		u64 last_reflink_trans;
+
+		/*
+		 * In case this a root stub inode (BTRFS_INODE_ROOT_STUB flag set),
+		 * the ID of that root.
+		 */
+		u64 ref_root_id;
+	};
 
 	/* Backwards incompatible flags, lower half of inode_item::flags  */
 	u32 flags;
@@ -387,11 +395,19 @@ static inline u64 btrfs_ino(const struct btrfs_inode *inode)
 static inline void btrfs_get_inode_key(const struct btrfs_inode *inode,
 				       struct btrfs_key *key)
 {
-	key->objectid = inode->objectid;
+	key->objectid = btrfs_ino(inode);
 	key->type = BTRFS_INODE_ITEM_KEY;
 	key->offset = 0;
 }
 
+static inline void btrfs_set_inode_number(struct btrfs_inode *inode, u64 ino)
+{
+#if BITS_PER_LONG == 32
+	inode->objectid = ino;
+#endif
+	inode->vfs_inode.i_ino = ino;
+}
+
 static inline void btrfs_i_size_write(struct btrfs_inode *inode, u64 size)
 {
 	i_size_write(&inode->vfs_inode, size);

fs/btrfs/disk-io.c

@@ -1928,7 +1928,7 @@ static int btrfs_init_btree_inode(struct super_block *sb)
 	if (!inode)
 		return -ENOMEM;
 
-	inode->i_ino = BTRFS_BTREE_INODE_OBJECTID;
+	btrfs_set_inode_number(BTRFS_I(inode), BTRFS_BTREE_INODE_OBJECTID);
 	set_nlink(inode, 1);
 	/*
 	 * we set the i_size on the btree inode to the max possible int.
@@ -1944,7 +1944,6 @@ static int btrfs_init_btree_inode(struct super_block *sb)
 	extent_map_tree_init(&BTRFS_I(inode)->extent_tree);
 
 	BTRFS_I(inode)->root = btrfs_grab_root(fs_info->tree_root);
-	BTRFS_I(inode)->objectid = BTRFS_BTREE_INODE_OBJECTID;
 	set_bit(BTRFS_INODE_DUMMY, &BTRFS_I(inode)->runtime_flags);
 	__insert_inode_hash(inode, hash);
 	fs_info->btree_inode = inode;

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);
 

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)

fs/btrfs/ioctl.c

@@ -1918,7 +1918,7 @@ static int btrfs_search_path_in_tree_user(struct mnt_idmap *idmap,
 {
 	struct btrfs_fs_info *fs_info = BTRFS_I(inode)->root->fs_info;
 	struct super_block *sb = inode->i_sb;
-	u64 upper_limit = BTRFS_I(inode)->objectid;
+	u64 upper_limit = btrfs_ino(BTRFS_I(inode));
 	u64 treeid = btrfs_root_id(BTRFS_I(inode)->root);
 	u64 dirid = args->dirid;
 	unsigned long item_off;
@@ -2140,7 +2140,7 @@ static int btrfs_ioctl_ino_lookup_user(struct file *file, void __user *argp)
 	inode = file_inode(file);
 
 	if (args->dirid == BTRFS_FIRST_FREE_OBJECTID &&
-	    BTRFS_I(inode)->objectid != BTRFS_FIRST_FREE_OBJECTID) {
+	    btrfs_ino(BTRFS_I(inode)) != BTRFS_FIRST_FREE_OBJECTID) {
 		/*
 		 * The subvolume does not exist under fd with which this is
 		 * called

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;