Btrfs: Make free space cache code generic

So we can re-use the code to cache free inode numbers.

The change is quite straightforward. Two new structures are introduced.

- struct btrfs_free_space_ctl

  We move those variables that are used for caching free space from
  struct btrfs_block_group_cache to this new struct.

- struct btrfs_free_space_op

  We do block group specific work (e.g. calculation of extents threshold)
  through functions registered in this struct.

And then we can remove references to struct btrfs_block_group_cache.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>

fs/btrfs/ctree.h

@@ -830,9 +830,6 @@ struct btrfs_block_group_cache {
 	u64 bytes_super;
 	u64 flags;
 	u64 sectorsize;
-	int extents_thresh;
-	int free_extents;
-	int total_bitmaps;
 	unsigned int ro:1;
 	unsigned int dirty:1;
 	unsigned int iref:1;
@@ -847,9 +844,7 @@ struct btrfs_block_group_cache {
 	struct btrfs_space_info *space_info;
 
 	/* free space cache stuff */
-	spinlock_t tree_lock;
-	struct rb_root free_space_offset;
-	u64 free_space;
+	struct btrfs_free_space_ctl *free_space_ctl;
 
 	/* block group cache stuff */
 	struct rb_node cache_node;

fs/btrfs/extent-tree.c

@@ -105,6 +105,7 @@ void btrfs_put_block_group(struct btrfs_block_group_cache *cache)
 		WARN_ON(cache->pinned > 0);
 		WARN_ON(cache->reserved > 0);
 		WARN_ON(cache->reserved_pinned > 0);
+		kfree(cache->free_space_ctl);
 		kfree(cache);
 	}
 }
@@ -4893,7 +4894,7 @@ wait_block_group_cache_progress(struct btrfs_block_group_cache *cache,
 		return 0;
 
 	wait_event(caching_ctl->wait, block_group_cache_done(cache) ||
-		   (cache->free_space >= num_bytes));
+		   (cache->free_space_ctl->free_space >= num_bytes));
 
 	put_caching_control(caching_ctl);
 	return 0;
@@ -8551,25 +8552,23 @@ int btrfs_read_block_groups(struct btrfs_root *root)
 			ret = -ENOMEM;
 			goto error;
 		}
+		cache->free_space_ctl = kzalloc(sizeof(*cache->free_space_ctl),
+						GFP_NOFS);
+		if (!cache->free_space_ctl) {
+			kfree(cache);
+			ret = -ENOMEM;
+			goto error;
+		}
 
 		atomic_set(&cache->count, 1);
 		spin_lock_init(&cache->lock);
-		spin_lock_init(&cache->tree_lock);
 		cache->fs_info = info;
 		INIT_LIST_HEAD(&cache->list);
 		INIT_LIST_HEAD(&cache->cluster_list);
 
 		if (need_clear)
 			cache->disk_cache_state = BTRFS_DC_CLEAR;
 
-		/*
-		 * we only want to have 32k of ram per block group for keeping
-		 * track of free space, and if we pass 1/2 of that we want to
-		 * start converting things over to using bitmaps
-		 */
-		cache->extents_thresh = ((1024 * 32) / 2) /
-			sizeof(struct btrfs_free_space);
-
 		read_extent_buffer(leaf, &cache->item,
 				   btrfs_item_ptr_offset(leaf, path->slots[0]),
 				   sizeof(cache->item));
@@ -8580,6 +8579,8 @@ int btrfs_read_block_groups(struct btrfs_root *root)
 		cache->flags = btrfs_block_group_flags(&cache->item);
 		cache->sectorsize = root->sectorsize;
 
+		btrfs_init_free_space_ctl(cache);
+
 		/*
 		 * We need to exclude the super stripes now so that the space
 		 * info has super bytes accounted for, otherwise we'll think
@@ -8666,26 +8667,26 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans,
 	cache = kzalloc(sizeof(*cache), GFP_NOFS);
 	if (!cache)
 		return -ENOMEM;
+	cache->free_space_ctl = kzalloc(sizeof(*cache->free_space_ctl),
+					GFP_NOFS);
+	if (!cache->free_space_ctl) {
+		kfree(cache);
+		return -ENOMEM;
+	}
 
 	cache->key.objectid = chunk_offset;
 	cache->key.offset = size;
 	cache->key.type = BTRFS_BLOCK_GROUP_ITEM_KEY;
 	cache->sectorsize = root->sectorsize;
 	cache->fs_info = root->fs_info;
 
-	/*
-	 * we only want to have 32k of ram per block group for keeping track
-	 * of free space, and if we pass 1/2 of that we want to start
-	 * converting things over to using bitmaps
-	 */
-	cache->extents_thresh = ((1024 * 32) / 2) /
-		sizeof(struct btrfs_free_space);
 	atomic_set(&cache->count, 1);
 	spin_lock_init(&cache->lock);
-	spin_lock_init(&cache->tree_lock);
 	INIT_LIST_HEAD(&cache->list);
 	INIT_LIST_HEAD(&cache->cluster_list);
 
+	btrfs_init_free_space_ctl(cache);
+
 	btrfs_set_block_group_used(&cache->item, bytes_used);
 	btrfs_set_block_group_chunk_objectid(&cache->item, chunk_objectid);
 	cache->flags = type;