diff --git a/fs/xfs/scrub/rtbitmap_repair.c b/fs/xfs/scrub/rtbitmap_repair.c
index dc64902d6c25..442a17bf9720 100644
--- a/fs/xfs/scrub/rtbitmap_repair.c
+++ b/fs/xfs/scrub/rtbitmap_repair.c
@@ -36,6 +36,24 @@
 
 /* rt bitmap content repairs */
 
+/*
+ * Reserve enough blocks to write out a completely new bitmap file, plus twice
+ * as many blocks as we would need if we can only allocate one block per data
+ * fork mapping.  This should cover the preallocation of the temporary file and
+ * exchanging the extent mappings.
+ *
+ * We cannot use xfs_exchmaps_estimate because we have not yet constructed the
+ * replacement bitmap and therefore do not know how many extents it will use.
+ * By the time we do, we will have a dirty transaction (which we cannot drop
+ * because we cannot drop the rtbitmap ILOCK) and cannot ask for more
+ * reservation.
+ */
+static inline unsigned long long
+xrep_rtbitmap_calc_blocks(struct xfs_mount *mp, unsigned long long blocks)
+{
+	return blocks + (xfs_bmbt_calc_size(mp, blocks) * 2);
+}
+
 /* Set up to repair the realtime bitmap for this group. */
 int
 xrep_setup_rtbitmap(
@@ -56,20 +74,7 @@ xrep_setup_rtbitmap(
 	if (error)
 		return error;
 
-	/*
-	 * Reserve enough blocks to write out a completely new bitmap file,
-	 * plus twice as many blocks as we would need if we can only allocate
-	 * one block per data fork mapping.  This should cover the
-	 * preallocation of the temporary file and exchanging the extent
-	 * mappings.
-	 *
-	 * We cannot use xfs_exchmaps_estimate because we have not yet
-	 * constructed the replacement bitmap and therefore do not know how
-	 * many extents it will use.  By the time we do, we will have a dirty
-	 * transaction (which we cannot drop because we cannot drop the
-	 * rtbitmap ILOCK) and cannot ask for more reservation.
-	 */
-	blocks += xfs_bmbt_calc_size(mp, blocks) * 2;
+	blocks = xrep_rtbitmap_calc_blocks(mp, mp->m_sb.sb_rbmblocks);
 	if (blocks > UINT_MAX)
 		return -EOPNOTSUPP;
 
@@ -512,7 +517,7 @@ xrep_rtbitmap(
 	struct xchk_rtbitmap	*rtb = sc->buf;
 	struct xfs_mount	*mp = sc->mp;
 	struct xfs_group	*xg = rtg_group(sc->sr.rtg);
-	unsigned long long	blocks = 0;
+	unsigned long long	blocks;
 	unsigned int		busy_gen;
 	int			error;
 
@@ -532,15 +537,20 @@ xrep_rtbitmap(
 	 * figure out if we need to adjust the block reservation in the
 	 * transaction.
 	 */
-	blocks = xfs_bmbt_calc_size(mp, rtb->rbmblocks);
+	blocks = xrep_rtbitmap_calc_blocks(mp, rtb->rbmblocks);
 	if (blocks > UINT_MAX)
 		return -EOPNOTSUPP;
 	if (blocks > rtb->resblks) {
-		error = xfs_trans_reserve_more(sc->tp, blocks, 0);
+		uint64_t	delta = blocks - rtb->resblks;
+
+		if (delta > UINT_MAX)
+			return -EOPNOTSUPP;
+
+		error = xfs_trans_reserve_more(sc->tp, delta, 0);
 		if (error)
 			return error;
 
-		rtb->resblks += blocks;
+		rtb->resblks += delta;
 	}
 
 	/* Fix inode core and forks. */