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>

fs/btrfs/btrfs_inode.h

@@ -225,12 +225,21 @@ struct btrfs_inode {
 		u64 last_dir_index_offset;
 	};
 
+	union {
 		/*
 		 * Total number of bytes pending defrag, used by stat to check whether
 		 * it needs COW. Protected by 'lock'.
+		 * Used by inodes other than the data relocation inode.
 		 */
 		u64 defrag_bytes;
 
+		/*
+		 * Logical address of the block group being relocated.
+		 * Used only by the data relocation inode.
+		 */
+		u64 reloc_block_group_start;
+	};
+
 	/*
 	 * The size of the file stored in the metadata on disk.  data=ordered
 	 * means the in-memory i_size might be larger than the size on disk
@@ -238,13 +247,22 @@ struct btrfs_inode {
 	 */
 	u64 disk_i_size;
 
+	union {
 		/*
-	 * If this is a directory then index_cnt is the counter for the index
-	 * number for new files that are created. For an empty directory, this
-	 * must be initialized to BTRFS_DIR_START_INDEX.
+		 * If this is a directory then index_cnt is the counter for the
+		 * index number for new files that are created. For an empty
+		 * directory, this must be initialized to BTRFS_DIR_START_INDEX.
 		 */
 		u64 index_cnt;
 
+		/*
+		 * If this is not a directory, this is the number of bytes
+		 * outstanding that are going to need csums. This is used in
+		 * ENOSPC accounting. Protected by 'lock'.
+		 */
+		u64 csum_bytes;
+	};
+
 	/* Cache the directory index number to speed the dir/file remove */
 	u64 dir_index;
 
@@ -266,12 +284,6 @@ struct btrfs_inode {
 	 */
 	u64 last_reflink_trans;
 
-	/*
-	 * Number of bytes outstanding that are going to need csums.  This is
-	 * used in ENOSPC accounting. Protected by 'lock'.
-	 */
-	u64 csum_bytes;
-
 	/* Backwards incompatible flags, lower half of inode_item::flags  */
 	u32 flags;
 	/* Read-only compatibility flags, upper half of inode_item::flags */

fs/btrfs/delayed-inode.c

@@ -1914,6 +1914,7 @@ int btrfs_fill_inode(struct inode *inode, u32 *rdev)
 	BTRFS_I(inode)->i_otime_nsec = btrfs_stack_timespec_nsec(&inode_item->otime);
 
 	inode->i_generation = BTRFS_I(inode)->generation;
+	if (S_ISDIR(inode->i_mode))
 		BTRFS_I(inode)->index_cnt = (u64)-1;
 
 	mutex_unlock(&delayed_node->mutex);

fs/btrfs/inode.c

@@ -3856,7 +3856,9 @@ static int btrfs_read_locked_inode(struct inode *inode,
 	inode->i_rdev = 0;
 	rdev = btrfs_inode_rdev(leaf, inode_item);
 
+	if (S_ISDIR(inode->i_mode))
 		BTRFS_I(inode)->index_cnt = (u64)-1;
+
 	btrfs_inode_split_flags(btrfs_inode_flags(leaf, inode_item),
 				&BTRFS_I(inode)->flags, &BTRFS_I(inode)->ro_flags);
 
@@ -6268,8 +6270,10 @@ int btrfs_create_new_inode(struct btrfs_trans_handle *trans,
 		if (ret)
 			goto out;
 	}
-	/* index_cnt is ignored for everything but a dir. */
+
+	if (S_ISDIR(inode->i_mode))
 		BTRFS_I(inode)->index_cnt = BTRFS_DIR_START_INDEX;
+
 	BTRFS_I(inode)->generation = trans->transid;
 	inode->i_generation = BTRFS_I(inode)->generation;
 
@@ -8435,8 +8439,12 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
 	ei->disk_i_size = 0;
 	ei->flags = 0;
 	ei->ro_flags = 0;
+	/*
+	 * ->index_cnt will be properly initialized later when creating a new
+	 * inode (btrfs_create_new_inode()) or when reading an existing inode
+	 * from disk (btrfs_read_locked_inode()).
+	 */
 	ei->csum_bytes = 0;
-	ei->index_cnt = (u64)-1;
 	ei->dir_index = 0;
 	ei->last_unlink_trans = 0;
 	ei->last_reflink_trans = 0;
@@ -8511,8 +8519,9 @@ void btrfs_destroy_inode(struct inode *vfs_inode)
 	if (!S_ISDIR(vfs_inode->i_mode)) {
 		WARN_ON(inode->delalloc_bytes);
 		WARN_ON(inode->new_delalloc_bytes);
-	}
 		WARN_ON(inode->csum_bytes);
+	}
+	if (!root || !btrfs_is_data_reloc_root(root))
 		WARN_ON(inode->defrag_bytes);
 
 	/*

fs/btrfs/relocation.c

@@ -962,7 +962,7 @@ static int get_new_location(struct inode *reloc_inode, u64 *new_bytenr,
 	if (!path)
 		return -ENOMEM;
 
-	bytenr -= BTRFS_I(reloc_inode)->index_cnt;
+	bytenr -= BTRFS_I(reloc_inode)->reloc_block_group_start;
 	ret = btrfs_lookup_file_extent(NULL, root, path,
 			btrfs_ino(BTRFS_I(reloc_inode)), bytenr, 0);
 	if (ret < 0)
@@ -2797,7 +2797,7 @@ static noinline_for_stack int prealloc_file_extent_cluster(
 	u64 alloc_hint = 0;
 	u64 start;
 	u64 end;
-	u64 offset = inode->index_cnt;
+	u64 offset = inode->reloc_block_group_start;
 	u64 num_bytes;
 	int nr;
 	int ret = 0;
@@ -2951,7 +2951,7 @@ static int relocate_one_folio(struct inode *inode, struct file_ra_state *ra,
 			      int *cluster_nr, unsigned long index)
 {
 	struct btrfs_fs_info *fs_info = inode_to_fs_info(inode);
-	u64 offset = BTRFS_I(inode)->index_cnt;
+	u64 offset = BTRFS_I(inode)->reloc_block_group_start;
 	const unsigned long last_index = (cluster->end - offset) >> PAGE_SHIFT;
 	gfp_t mask = btrfs_alloc_write_mask(inode->i_mapping);
 	struct folio *folio;
@@ -3086,7 +3086,7 @@ static int relocate_one_folio(struct inode *inode, struct file_ra_state *ra,
 static int relocate_file_extent_cluster(struct inode *inode,
 					const struct file_extent_cluster *cluster)
 {
-	u64 offset = BTRFS_I(inode)->index_cnt;
+	u64 offset = BTRFS_I(inode)->reloc_block_group_start;
 	unsigned long index;
 	unsigned long last_index;
 	struct file_ra_state *ra;
@@ -3915,7 +3915,7 @@ static noinline_for_stack struct inode *create_reloc_inode(
 		inode = NULL;
 		goto out;
 	}
-	BTRFS_I(inode)->index_cnt = group->start;
+	BTRFS_I(inode)->reloc_block_group_start = group->start;
 
 	ret = btrfs_orphan_add(trans, BTRFS_I(inode));
 out:
@@ -4395,7 +4395,7 @@ int btrfs_reloc_clone_csums(struct btrfs_ordered_extent *ordered)
 {
 	struct btrfs_inode *inode = BTRFS_I(ordered->inode);
 	struct btrfs_fs_info *fs_info = inode->root->fs_info;
-	u64 disk_bytenr = ordered->file_offset + inode->index_cnt;
+	u64 disk_bytenr = ordered->file_offset + inode->reloc_block_group_start;
 	struct btrfs_root *csum_root = btrfs_csum_root(fs_info, disk_bytenr);
 	LIST_HEAD(list);
 	int ret;

fs/btrfs/tree-log.c

@@ -1644,6 +1644,7 @@ static noinline int fixup_inode_link_count(struct btrfs_trans_handle *trans,
 		if (ret)
 			goto out;
 	}
+	if (S_ISDIR(inode->i_mode))
 		BTRFS_I(inode)->index_cnt = (u64)-1;
 
 	if (inode->i_nlink == 0) {