1 files changed, 14 insertions, 9 deletions

diff --git a/src/third_party/wiredtiger/src/block/block_ckpt.c b/src/third_party/wiredtiger/src/block/block_ckpt.c
index 292917be5d6..7480ddfca2b 100644
--- a/src/third_party/wiredtiger/src/block/block_ckpt.c
+++ b/src/third_party/wiredtiger/src/block/block_ckpt.c
@@ -661,14 +661,20 @@ static int
 __ckpt_add_blkmod_entry(
   WT_SESSION_IMPL *session, WT_BLOCK_MODS *blk_mod, wt_off_t offset, wt_off_t len)
 {
-    uint64_t end, start;
-    uint32_t end_buf_bytes, end_rdup_bytes;
+    uint64_t end_bit, start_bit;
+    uint32_t end_buf_bytes, end_rdup_bits, end_rdup_bytes;
 
     WT_ASSERT(session, blk_mod->granularity != 0);
-    start = (uint64_t)offset / blk_mod->granularity;
-    end = (uint64_t)(offset + len) / blk_mod->granularity;
-    WT_ASSERT(session, end < UINT32_MAX);
-    end_rdup_bytes = WT_MAX(__wt_rduppo2((uint32_t)end, 8), WT_BLOCK_MODS_LIST_MIN);
+    /*
+     * Figure out how the starting and ending bits based on the granularity and our offset and
+     * length.
+     */
+    start_bit = (uint64_t)offset / blk_mod->granularity;
+    end_bit = (uint64_t)(offset + len - 1) / blk_mod->granularity;
+    WT_ASSERT(session, end_bit < UINT32_MAX);
+    /* We want to grow the bitmap by 64 bits, or 8 bytes at a time. */
+    end_rdup_bits = WT_MAX(__wt_rduppo2((uint32_t)end_bit, 64), WT_BLOCK_MODS_LIST_MIN);
+    end_rdup_bytes = end_rdup_bits >> 3;
     end_buf_bytes = (uint32_t)blk_mod->nbits >> 3;
     /*
      * We are doing a lot of shifting. Make sure that the number of bytes we end up with is a
@@ -687,11 +693,10 @@ __ckpt_add_blkmod_entry(
             memset(
               (uint8_t *)blk_mod->bitstring.mem + end_buf_bytes, 0, end_rdup_bytes - end_buf_bytes);
         }
-        blk_mod->nbits = end_rdup_bytes << 3;
+        blk_mod->nbits = end_rdup_bits;
     }
-
     /* Set all the bits needed to record this offset/length pair. */
-    __bit_nset(blk_mod->bitstring.mem, start, end);
+    __bit_nset(blk_mod->bitstring.mem, start_bit, end_bit);
     return (0);
 }