net/9p: distinguish zero-copy requests

Add boolean `zc` member to struct p9_fcall to distinguish zero-copy
messages (not using the linear `sdata` buffer for message payload) from
regular messages (which do copy message payload to `sdata` before being
further processed).

This new member is appended to end of structure to avoid inserting huge
padding in generated layout.

Link: https://lkml.kernel.org/r/8f2a5c12a446c3b544da64e0b1550e1fb2d6f972.1669144861.git.linux_oss@crudebyte.com
Signed-off-by: Christian Schoenebeck <[email protected]>
Tested-by: Stefano Stabellini <[email protected]>
Signed-off-by: Dominique Martinet <[email protected]>

Author: cschoenebeck

include/net/9p/9p.h

@@ -531,6 +531,7 @@ struct p9_rstatfs {
  * @offset: used by marshalling routines to track current position in buffer
  * @capacity: used by marshalling routines to track total malloc'd capacity
  * @sdata: payload
+ * @zc: whether zero-copy is used
  *
  * &p9_fcall represents the structure for all 9P RPC
  * transactions.  Requests are packaged into fcalls, and reponses
@@ -549,6 +550,7 @@ struct p9_fcall {
 
 	struct kmem_cache *cache;
 	u8 *sdata;
+	bool zc;
 };
 
 int p9_errstr2errno(char *errstr, int len);

net/9p/client.c

@@ -685,6 +685,9 @@ p9_client_rpc(struct p9_client *c, int8_t type, const char *fmt, ...)
 	if (IS_ERR(req))
 		return req;
 
+	req->tc.zc = false;
+	req->rc.zc = false;
+
 	if (signal_pending(current)) {
 		sigpending = 1;
 		clear_thread_flag(TIF_SIGPENDING);
@@ -783,6 +786,9 @@ static struct p9_req_t *p9_client_zc_rpc(struct p9_client *c, int8_t type,
 	if (IS_ERR(req))
 		return req;
 
+	req->tc.zc = true;
+	req->rc.zc = true;
+
 	if (signal_pending(current)) {
 		sigpending = 1;
 		clear_thread_flag(TIF_SIGPENDING);