[PATCH net-next 1/5] mptcp: reduce 'overhead' from u16 to u8

["Matthieu Baerts (NGI0)" <matttbe@kernel.org>]

From: Gang Yan <yangang@kylinos.cn>

The 'overhead' in struct mptcp_data_frag can safely use u8, as it
represents 'alignment + sizeof(mptcp_data_frag)'. With a maximum
alignment of 7('ALIGN(1, sizeof(long)) - 1'), the overhead is at most
47, well below U8_MAX and validated with BUILD_BUG_ON().

This patch also adds a field named 'unused' for further extensions.

Signed-off-by: Gang Yan <yangang@kylinos.cn>
Reviewed-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
---
 net/mptcp/protocol.c | 6 ++++++
 net/mptcp/protocol.h | 3 ++-
 2 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 239fb9e75c7c..79315e575d07 100644
--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -4616,6 +4616,12 @@ void __init mptcp_proto_init(void)
 	inet_register_protosw(&mptcp_protosw);
 
 	BUILD_BUG_ON(sizeof(struct mptcp_skb_cb) > sizeof_field(struct sk_buff, cb));
+
+	/* struct mptcp_data_frag: 'overhead' corresponds to the alignment
+	 * (ALIGN(1, sizeof(long)) - 1, so 8-1) + the struct's size
+	 */
+	BUILD_BUG_ON(ALIGN(1, sizeof(long)) - 1 + sizeof(struct mptcp_data_frag)
+		     > U8_MAX);
 }
 
 #if IS_ENABLED(CONFIG_MPTCP_IPV6)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 0bd1ee860316..02031007100b 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -263,7 +263,8 @@ struct mptcp_data_frag {
 	u64 data_seq;
 	u16 data_len;
 	u16 offset;
-	u16 overhead;
+	u8 overhead;
+	u8 __unused;
 	u16 already_sent;
 	struct page *page;
 };

-- 
2.53.0