btrfs: change the minimum global reserve size

It made sense to have the global reserve set at 16M in the past, but
since it is used less nowadays set the minimum size to the number of
items we'll need to update the main trees we update during a transaction
commit, plus some slop area so we can do unlinks if we need to.

In practice this doesn't affect normal file systems, but for xfstests
where we do things like fill up a fs and then rm * it can fall over in
weird ways.  This enables us for more sane behavior at extremely small
file system sizes.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-rsv.c

@@ -259,6 +259,7 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
 	struct btrfs_block_rsv *block_rsv = &fs_info->global_block_rsv;
 	struct btrfs_space_info *sinfo = block_rsv->space_info;
 	u64 num_bytes;
+	unsigned min_items;
 
 	/*
 	 * The global block rsv is based on the size of the extent tree, the
@@ -268,7 +269,26 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
 	num_bytes = btrfs_root_used(&fs_info->extent_root->root_item) +
 		btrfs_root_used(&fs_info->csum_root->root_item) +
 		btrfs_root_used(&fs_info->tree_root->root_item);
-	num_bytes = max_t(u64, num_bytes, SZ_16M);
+
+	/*
+	 * We at a minimum are going to modify the csum root, the tree root, and
+	 * the extent root.
+	 */
+	min_items = 3;
+
+	/*
+	 * But we also want to reserve enough space so we can do the fallback
+	 * global reserve for an unlink, which is an additional 5 items (see the
+	 * comment in __unlink_start_trans for what we're modifying.)
+	 *
+	 * But we also need space for the delayed ref updates from the unlink,
+	 * so its 10, 5 for the actual operation, and 5 for the delayed ref
+	 * updates.
+	 */
+	min_items += 10;
+
+	num_bytes = max_t(u64, num_bytes,
+			  btrfs_calc_insert_metadata_size(fs_info, min_items));
 
 	spin_lock(&sinfo->lock);
 	spin_lock(&block_rsv->lock);