diff --git a/fs/btrfs/extent_map.c b/fs/btrfs/extent_map.c
index a6d8368ed0ed..8c017c4105f2 100644
--- a/fs/btrfs/extent_map.c
+++ b/fs/btrfs/extent_map.c
@@ -843,7 +843,7 @@ void btrfs_drop_extent_map_range(struct btrfs_inode *inode, u64 start, u64 end,
 					split->block_len = em->block_len;
 					split->orig_start = em->orig_start;
 				} else {
-					const u64 diff = start + len - em->start;
+					const u64 diff = end - em->start;
 
 					split->block_len = split->len;
 					split->block_start += diff;
diff --git a/fs/btrfs/tests/extent-map-tests.c b/fs/btrfs/tests/extent-map-tests.c
index 29bdd08b241f..bf85c75ee722 100644
--- a/fs/btrfs/tests/extent-map-tests.c
+++ b/fs/btrfs/tests/extent-map-tests.c
@@ -826,6 +826,11 @@ static int test_case_7(void)
 		goto out;
 	}
 
+	if (em->block_start != SZ_32K + SZ_4K) {
+		test_err("em->block_start is %llu, expected 36K", em->block_start);
+		goto out;
+	}
+
 	free_extent_map(em);
 
 	read_lock(&em_tree->lock);