diff --git a/include/uapi/linux/bcache.h b/include/uapi/linux/bcache.h
index 47df2db2e727653b281101e143b31031ac1f89eb..0ef984ea515a81dcf7382f27ed7f3d15081c3e2c 100644
--- a/include/uapi/linux/bcache.h
+++ b/include/uapi/linux/bcache.h
@@ -215,8 +215,13 @@ struct cache_sb_disk {
 	__le64			d[SB_JOURNAL_BUCKETS];	/* journal buckets */
 };
 
+/*
+ * This is for in-memory bcache super block.
+ * NOTE: cache_sb is NOT exactly mapping to cache_sb_disk, the member
+ *       size, ordering and even whole struct size may be different
+ *       from cache_sb_disk.
+ */
 struct cache_sb {
-	__u64			csum;
 	__u64			offset;	/* sector where this sb was written */
 	__u64			version;
 
@@ -236,8 +241,6 @@ struct cache_sb {
 	__u64			feature_incompat;
 	__u64			feature_ro_compat;
 
-	__u64			pad[5];
-
 	union {
 	struct {
 		/* Cache devices */
@@ -245,7 +248,6 @@ struct cache_sb {
 
 		__u16		block_size;	/* sectors */
 		__u16		bucket_size;	/* sectors */
-
 		__u16		nr_in_set;
 		__u16		nr_this_dev;
 	};