btrfs: remove dead create_space_info calls

Since commit 2be12ef79 (btrfs: Separate space_info create/update), we've
separated out the creation and updating of the space info structures.
That commit was a straightforward refactoring of the two parts of
update_space_info, but we can go a step further.  Since commits
c59021f84 (Btrfs: fix OOPS of empty filesystem after balance) and
b742bb82f (Btrfs: Link block groups of different raid types), we know
that the space_info structures will be created at mount and there will
only ever be, at most, three of them.

This patch cleans out the create_space_info calls after __find_space_info
returns NULL since __find_space_info *can't* return NULL.

The initial cause for reviewing this was the kobject_add calls from
create_space_info occuring in sites where fs-reclaim wasn't allowed.  Now
we are certain they occur only early in the mount process and are safe.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -954,9 +954,9 @@ struct btrfs_fs_info {
 	struct btrfs_fs_devices *fs_devices;
 
 	/*
-	 * the space_info list is almost entirely read only.  It only changes
-	 * when we add a new raid type to the FS, and that happens
-	 * very rarely.  RCU is used to protect it.
+	 * The space_info list is effectively read only after initial
+	 * setup.  It is populated at mount time and cleaned up after
+	 * all block groups are removed.  RCU is used to protect it.
 	 */
 	struct list_head space_info;
 

fs/btrfs/extent-tree.c

@@ -4606,11 +4606,7 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
 		return -ENOSPC;
 
 	space_info = __find_space_info(fs_info, flags);
-	if (!space_info) {
-		ret = create_space_info(fs_info, flags, &space_info);
-		if (ret)
-			return ret;
-	}
+	ASSERT(space_info);
 
 again:
 	spin_lock(&space_info->lock);
@@ -10265,15 +10261,7 @@ int btrfs_make_block_group(struct btrfs_trans_handle *trans,
 	 * with its ->space_info set.
 	 */
 	cache->space_info = __find_space_info(fs_info, cache->flags);
-	if (!cache->space_info) {
-		ret = create_space_info(fs_info, cache->flags,
-				       &cache->space_info);
-		if (ret) {
-			btrfs_remove_free_space_cache(cache);
-			btrfs_put_block_group(cache);
-			return ret;
-		}
-	}
+	ASSERT(cache->space_info);
 
 	ret = btrfs_add_block_group_cache(fs_info, cache);
 	if (ret) {