btrfs: use a constant for the number of metadata units needed for an unlink

Instead of hard coding the number of metadata units for an unlink operation
in a couple places, define a macro and use it instead. This eliminates the
problem of one place getting out of sync with the other, such as recently
fixed by the previous patch in the series ("btrfs: fix calculation of the
global block reserve's size").
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/block-rsv.c

@@ -350,14 +350,15 @@ void btrfs_update_global_block_rsv(struct btrfs_fs_info *fs_info)
 
 	/*
 	 * But we also want to reserve enough space so we can do the fallback
-	 * global reserve for an unlink, which is an additional 6 items (see the
-	 * comment in __unlink_start_trans for what we're modifying.)
+	 * global reserve for an unlink, which is an additional
+	 * BTRFS_UNLINK_METADATA_UNITS items.
 	 *
 	 * But we also need space for the delayed ref updates from the unlink,
-	 * so its 12, 6 for the actual operation, and 6 for the delayed ref
-	 * updates.
+	 * so it's BTRFS_UNLINK_METADATA_UNITS * 2, BTRFS_UNLINK_METADATA_UNITS
+	 * for the actual operation, and BTRFS_UNLINK_METADATA_UNITS more for
+	 * the delayed ref updates.
 	 */
-	min_items += 12;
+	min_items += BTRFS_UNLINK_METADATA_UNITS * 2;
 
 	num_bytes = max_t(u64, num_bytes,
 			  btrfs_calc_insert_metadata_size(fs_info, min_items));

fs/btrfs/fs.h

@@ -24,6 +24,18 @@
 #define BTRFS_SUPER_INFO_SIZE			4096
 static_assert(sizeof(struct btrfs_super_block) == BTRFS_SUPER_INFO_SIZE);
 
+/*
+ * Number of metadata items necessary for an unlink operation:
+ *
+ * 1 for the possible orphan item
+ * 1 for the dir item
+ * 1 for the dir index
+ * 1 for the inode ref
+ * 1 for the inode
+ * 1 for the parent inode
+ */
+#define BTRFS_UNLINK_METADATA_UNITS		6
+
 /*
  * The reserved space at the beginning of each device.  It covers the primary
  * super block and leaves space for potential use by other tools like

fs/btrfs/inode.c

@@ -4248,15 +4248,8 @@ static struct btrfs_trans_handle *__unlink_start_trans(struct btrfs_inode *dir)
 {
 	struct btrfs_root *root = dir->root;
 
-	/*
-	 * 1 for the possible orphan item
-	 * 1 for the dir item
-	 * 1 for the dir index
-	 * 1 for the inode ref
-	 * 1 for the inode
-	 * 1 for the parent inode
-	 */
-	return btrfs_start_transaction_fallback_global_rsv(root, 6);
+	return btrfs_start_transaction_fallback_global_rsv(root,
+						   BTRFS_UNLINK_METADATA_UNITS);
 }
 
 static int btrfs_unlink(struct inode *dir, struct dentry *dentry)