Btrfs: fix necessary chunk tree space calculation when allocating a chunk

When allocating a new chunk or removing one we need to update num_devs
device items and insert or remove a chunk item in the chunk tree, so
in the worst case the space needed in the chunk space_info is:

  btrfs_calc_trunc_metadata_size(chunk_root, num_devs) +
     btrfs_calc_trans_metadata_size(chunk_root, 1)

That is, in the worst case we need to cow num_devs paths and cow 1 other
path that can result in splitting every node and leaf, and each path
consisting of BTRFS_MAX_LEVEL - 1 nodes and 1 leaf. We were requiring
some additional chunk_root->nodesize * BTRFS_MAX_LEVEL * num_devs bytes,
which were unnecessary since updating the existing device items does
not result in splitting the nodes and leaf since after updating them
they remain with the same size.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/ctree.h

@@ -3518,8 +3518,7 @@ int btrfs_start_write_no_snapshoting(struct btrfs_root *root);
 void btrfs_end_write_no_snapshoting(struct btrfs_root *root);
 void check_system_chunk(struct btrfs_trans_handle *trans,
 			struct btrfs_root *root,
-			const u64 type,
-			const bool is_allocation);
+			const u64 type);
 /* ctree.c */
 int btrfs_bin_search(struct extent_buffer *eb, struct btrfs_key *key,
 		     int level, int *slot);

fs/btrfs/extent-tree.c

@@ -4116,8 +4116,7 @@ static u64 get_profile_num_devs(struct btrfs_root *root, u64 type)
  */
 void check_system_chunk(struct btrfs_trans_handle *trans,
 			struct btrfs_root *root,
-			u64 type,
-			const bool is_allocation)
+			u64 type)
 {
 	struct btrfs_space_info *info;
 	u64 left;
@@ -4141,11 +4140,8 @@ void check_system_chunk(struct btrfs_trans_handle *trans,
 	num_devs = get_profile_num_devs(root, type);
 
 	/* num_devs device items to update and 1 chunk item to add or remove */
-	if (is_allocation)
-		thresh = btrfs_calc_trans_metadata_size(root, num_devs + 1);
-	else
-		thresh = btrfs_calc_trans_metadata_size(root, num_devs) +
-			btrfs_calc_trunc_metadata_size(root, 1);
+	thresh = btrfs_calc_trunc_metadata_size(root, num_devs) +
+		btrfs_calc_trans_metadata_size(root, 1);
 
 	if (left < thresh && btrfs_test_opt(root, ENOSPC_DEBUG)) {
 		btrfs_info(root->fs_info, "left=%llu, need=%llu, flags=%llu",
@@ -4258,7 +4254,7 @@ again:
 	 * Check if we have enough space in SYSTEM chunk because we may need
 	 * to update devices.
 	 */
-	check_system_chunk(trans, extent_root, flags, true);
+	check_system_chunk(trans, extent_root, flags);
 
 	ret = btrfs_alloc_chunk(trans, extent_root, flags);
 	trans->allocating_chunk = false;
@@ -8926,7 +8922,7 @@ out:
 	if (cache->flags & BTRFS_BLOCK_GROUP_SYSTEM) {
 		alloc_flags = update_block_group_flags(root, cache->flags);
 		lock_chunks(root->fs_info->chunk_root);
-		check_system_chunk(trans, root, alloc_flags, true);
+		check_system_chunk(trans, root, alloc_flags);
 		unlock_chunks(root->fs_info->chunk_root);
 	}
 	mutex_unlock(&root->fs_info->ro_block_group_mutex);

fs/btrfs/volumes.c

@@ -2626,7 +2626,7 @@ int btrfs_remove_chunk(struct btrfs_trans_handle *trans,
 	}
 	map = (struct map_lookup *)em->bdev;
 	lock_chunks(root->fs_info->chunk_root);
-	check_system_chunk(trans, extent_root, map->type, false);
+	check_system_chunk(trans, extent_root, map->type);
 	unlock_chunks(root->fs_info->chunk_root);
 
 	for (i = 0; i < map->num_stripes; i++) {