btrfs: allocate btrfs_inode::file_extent_tree only without NO_HOLES

The file_extent_tree was added in 41a2ee75 ("btrfs: introduce
per-inode file extent tree") so we have an explicit mapping of the file
extents to know where it is safe to update i_size. When the feature
NO_HOLES is enabled, and it's been a mkfs default since 5.15, the tree
is not necessary.

To save some space in the inode, allocate the tree only when necessary.
This reduces size by 16 bytes from 1096 to 1080 on a x86_64 release
config.
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/btrfs_inode.h

@@ -109,9 +109,11 @@ struct btrfs_inode {
 
 	/*
 	 * Keep track of where the inode has extent items mapped in order to
-	 * make sure the i_size adjustments are accurate
+	 * make sure the i_size adjustments are accurate. Not required when the
+	 * filesystem is NO_HOLES, the status can't be set while mounted as
+	 * it's a mkfs-time feature.
 	 */
-	struct extent_io_tree file_extent_tree;
+	struct extent_io_tree *file_extent_tree;
 
 	/* held while logging the inode in tree-log.c */
 	struct mutex log_mutex;

fs/btrfs/extent-io-tree.c

@@ -962,6 +962,8 @@ int find_contiguous_extent_bit(struct extent_io_tree *tree, u64 start,
 	struct extent_state *state;
 	int ret = 1;
 
+	ASSERT(!btrfs_fs_incompat(extent_io_tree_to_fs_info(tree), NO_HOLES));
+
 	spin_lock(&tree->lock);
 	state = find_first_extent_bit_state(tree, start, bits);
 	if (state) {

fs/btrfs/file-item.c

@@ -59,7 +59,7 @@ void btrfs_inode_safe_disk_i_size_write(struct btrfs_inode *inode, u64 new_i_siz
 		goto out_unlock;
 	}
 
-	ret = find_contiguous_extent_bit(&inode->file_extent_tree, 0, &start,
+	ret = find_contiguous_extent_bit(inode->file_extent_tree, 0, &start,
 					 &end, EXTENT_DIRTY);
 	if (!ret && start == 0)
 		i_size = min(i_size, end + 1);
@@ -94,7 +94,7 @@ int btrfs_inode_set_file_extent_range(struct btrfs_inode *inode, u64 start,
 
 	if (btrfs_fs_incompat(inode->root->fs_info, NO_HOLES))
 		return 0;
-	return set_extent_bit(&inode->file_extent_tree, start, start + len - 1,
+	return set_extent_bit(inode->file_extent_tree, start, start + len - 1,
 			      EXTENT_DIRTY, NULL);
 }
 
@@ -123,7 +123,7 @@ int btrfs_inode_clear_file_extent_range(struct btrfs_inode *inode, u64 start,
 
 	if (btrfs_fs_incompat(inode->root->fs_info, NO_HOLES))
 		return 0;
-	return clear_extent_bit(&inode->file_extent_tree, start,
+	return clear_extent_bit(inode->file_extent_tree, start,
 				start + len - 1, EXTENT_DIRTY, NULL);
 }
 

fs/btrfs/inode.c

@@ -8479,10 +8479,20 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
 	struct btrfs_fs_info *fs_info = btrfs_sb(sb);
 	struct btrfs_inode *ei;
 	struct inode *inode;
+	struct extent_io_tree *file_extent_tree = NULL;
+
+	/* Self tests may pass a NULL fs_info. */
+	if (fs_info && !btrfs_fs_incompat(fs_info, NO_HOLES)) {
+		file_extent_tree = kmalloc(sizeof(struct extent_io_tree), GFP_KERNEL);
+		if (!file_extent_tree)
+			return NULL;
+	}
 
 	ei = alloc_inode_sb(sb, btrfs_inode_cachep, GFP_KERNEL);
-	if (!ei)
+	if (!ei) {
+		kfree(file_extent_tree);
 		return NULL;
+	}
 
 	ei->root = NULL;
 	ei->generation = 0;
@@ -8523,10 +8533,13 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
 	extent_io_tree_init(fs_info, &ei->io_tree, IO_TREE_INODE_IO);
 	ei->io_tree.inode = ei;
 
-	extent_io_tree_init(fs_info, &ei->file_extent_tree,
-			    IO_TREE_INODE_FILE_EXTENT);
-	/* Lockdep class is set only for the file extent tree. */
-	lockdep_set_class(&ei->file_extent_tree.lock, &file_extent_tree_class);
+	ei->file_extent_tree = file_extent_tree;
+	if (file_extent_tree) {
+		extent_io_tree_init(fs_info, ei->file_extent_tree,
+				    IO_TREE_INODE_FILE_EXTENT);
+		/* Lockdep class is set only for the file extent tree. */
+		lockdep_set_class(&ei->file_extent_tree->lock, &file_extent_tree_class);
+	}
 	mutex_init(&ei->log_mutex);
 	spin_lock_init(&ei->ordered_tree_lock);
 	ei->ordered_tree = RB_ROOT;
@@ -8543,12 +8556,14 @@ struct inode *btrfs_alloc_inode(struct super_block *sb)
 void btrfs_test_destroy_inode(struct inode *inode)
 {
 	btrfs_drop_extent_map_range(BTRFS_I(inode), 0, (u64)-1, false);
+	kfree(BTRFS_I(inode)->file_extent_tree);
 	kmem_cache_free(btrfs_inode_cachep, BTRFS_I(inode));
 }
 #endif
 
 void btrfs_free_inode(struct inode *inode)
 {
+	kfree(BTRFS_I(inode)->file_extent_tree);
 	kmem_cache_free(btrfs_inode_cachep, BTRFS_I(inode));
 }