Btrfs: remove unneeded field / smaller extent_map structure

We don't need to have an unsigned int field in the extent_map struct
to tell us whether the extent map is in the inode's extent_map tree or
not. We can use the rb_node struct field and the RB_CLEAR_NODE and
RB_EMPTY_NODE macros to achieve the same task.

This reduces sizeof(struct extent_map) from 152 bytes to 144 bytes (on a
64 bits system).
Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fb.com>

fs/btrfs/extent_io.c

@@ -2763,7 +2763,7 @@ __get_extent_map(struct inode *inode, struct page *page, size_t pg_offset,
 
 	if (em_cached && *em_cached) {
 		em = *em_cached;
-		if (em->in_tree && start >= em->start &&
+		if (extent_map_in_tree(em) && start >= em->start &&
 		    start < extent_map_end(em)) {
 			atomic_inc(&em->refs);
 			return em;

fs/btrfs/extent_map.c

@@ -51,7 +51,7 @@ struct extent_map *alloc_extent_map(void)
 	em = kmem_cache_zalloc(extent_map_cache, GFP_NOFS);
 	if (!em)
 		return NULL;
-	em->in_tree = 0;
+	RB_CLEAR_NODE(&em->rb_node);
 	em->flags = 0;
 	em->compress_type = BTRFS_COMPRESS_NONE;
 	em->generation = 0;
@@ -73,7 +73,7 @@ void free_extent_map(struct extent_map *em)
 		return;
 	WARN_ON(atomic_read(&em->refs) == 0);
 	if (atomic_dec_and_test(&em->refs)) {
-		WARN_ON(em->in_tree);
+		WARN_ON(extent_map_in_tree(em));
 		WARN_ON(!list_empty(&em->list));
 		kmem_cache_free(extent_map_cache, em);
 	}
@@ -99,8 +99,6 @@ static int tree_insert(struct rb_root *root, struct extent_map *em)
 		parent = *p;
 		entry = rb_entry(parent, struct extent_map, rb_node);
 
-		WARN_ON(!entry->in_tree);
-
 		if (em->start < entry->start)
 			p = &(*p)->rb_left;
 		else if (em->start >= extent_map_end(entry))
@@ -128,7 +126,6 @@ static int tree_insert(struct rb_root *root, struct extent_map *em)
 		if (end > entry->start && em->start < extent_map_end(entry))
 			return -EEXIST;
 
-	em->in_tree = 1;
 	rb_link_node(&em->rb_node, orig_parent, p);
 	rb_insert_color(&em->rb_node, root);
 	return 0;
@@ -153,8 +150,6 @@ static struct rb_node *__tree_search(struct rb_root *root, u64 offset,
 		prev = n;
 		prev_entry = entry;
 
-		WARN_ON(!entry->in_tree);
-
 		if (offset < entry->start)
 			n = n->rb_left;
 		else if (offset >= extent_map_end(entry))
@@ -240,12 +235,12 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 			em->len += merge->len;
 			em->block_len += merge->block_len;
 			em->block_start = merge->block_start;
-			merge->in_tree = 0;
 			em->mod_len = (em->mod_len + em->mod_start) - merge->mod_start;
 			em->mod_start = merge->mod_start;
 			em->generation = max(em->generation, merge->generation);
 
 			rb_erase(&merge->rb_node, &tree->map);
+			RB_CLEAR_NODE(&merge->rb_node);
 			free_extent_map(merge);
 		}
 	}
@@ -257,7 +252,7 @@ static void try_merge_map(struct extent_map_tree *tree, struct extent_map *em)
 		em->len += merge->len;
 		em->block_len += merge->block_len;
 		rb_erase(&merge->rb_node, &tree->map);
-		merge->in_tree = 0;
+		RB_CLEAR_NODE(&merge->rb_node);
 		em->mod_len = (merge->mod_start + merge->mod_len) - em->mod_start;
 		em->generation = max(em->generation, merge->generation);
 		free_extent_map(merge);
@@ -319,7 +314,7 @@ int unpin_extent_cache(struct extent_map_tree *tree, u64 start, u64 len,
 void clear_em_logging(struct extent_map_tree *tree, struct extent_map *em)
 {
 	clear_bit(EXTENT_FLAG_LOGGING, &em->flags);
-	if (em->in_tree)
+	if (extent_map_in_tree(em))
 		try_merge_map(tree, em);
 }
 
@@ -434,6 +429,6 @@ int remove_extent_mapping(struct extent_map_tree *tree, struct extent_map *em)
 	rb_erase(&em->rb_node, &tree->map);
 	if (!test_bit(EXTENT_FLAG_LOGGING, &em->flags))
 		list_del_init(&em->list);
-	em->in_tree = 0;
+	RB_CLEAR_NODE(&em->rb_node);
 	return ret;
 }

fs/btrfs/extent_map.h

@@ -33,7 +33,6 @@ struct extent_map {
 	unsigned long flags;
 	struct block_device *bdev;
 	atomic_t refs;
-	unsigned int in_tree;
 	unsigned int compress_type;
 	struct list_head list;
 };
@@ -44,6 +43,11 @@ struct extent_map_tree {
 	rwlock_t lock;
 };
 
+static inline int extent_map_in_tree(const struct extent_map *em)
+{
+	return !RB_EMPTY_NODE(&em->rb_node);
+}
+
 static inline u64 extent_map_end(struct extent_map *em)
 {
 	if (em->start + em->len < em->start)