mptcp: reduce 'overhead' from u16 to u8

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>
Link: https://patch.msgid.link/20260403-net-next-mptcp-msg_eor-misc-v1-1-b0b33bea3fed@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/mptcp/protocol.c b/net/mptcp/protocol.c
index 239fb9e75c7c2e97546de20f145c696b3c33f51b..79315e575d074b346dccbdc4f6c5d015e82bcd41 100644 (file)
--- 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 0bd1ee860316dd25e88beb1b9ba020abdc32f7cb..02031007100b2c61551235640daaa3eadd9f193b 100644 (file)
--- 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;
 };