Re: [PATCH V2 net-next 5/7] rds: zerocopy Tx support.

If the MSG_ZEROCOPY flag is specified with rds_sendmsg(), and,
if the SO_ZEROCOPY socket option has been set on the PF_RDS socket,
application pages sent down with rds_sendmsg() are pinned.

The pinning uses the accounting infrastructure added by
Commit a91dbff551a6 ("sock: ulimit on MSG_ZEROCOPY pages")

The payload bytes in the message may not be modified for the
duration that the message has been pinned. A multi-threaded
application using this infrastructure may thus need to be notified
about send-completion so that it can free/reuse the buffers
passed to rds_sendmsg(). Notification of send-completion will
identify each message-buffer by a cookie that the application
must specify as ancillary data to rds_sendmsg().
The ancillary data in this case has cmsg_level == SOL_RDS
and cmsg_type == RDS_CMSG_ZCOPY_COOKIE.

Signed-off-by: Sowmini Varadhan <redacted>
---
v2:
   - remove unused data_len argument to rds_rm_size;
   - unmap as necessary if we fail in the middle of zerocopy setup

  include/uapi/linux/rds.h |    1 +
  net/rds/message.c        |   51 +++++++++++++++++++++++++++++++++++++++++++++-
  net/rds/rds.h            |    3 +-
  net/rds/send.c           |   44 ++++++++++++++++++++++++++++++++++-----
  4 files changed, 91 insertions(+), 8 deletions(-)

diff --git a/include/uapi/linux/rds.h b/include/uapi/linux/rds.h
index e71d449..12e3bca 100644
--- a/include/uapi/linux/rds.h
+++ b/include/uapi/linux/rds.h

@@ -103,6 +103,7 @@
  #define RDS_CMSG_MASKED_ATOMIC_FADD	8
  #define RDS_CMSG_MASKED_ATOMIC_CSWP	9
  #define RDS_CMSG_RXPATH_LATENCY		11
+#define	RDS_CMSG_ZCOPY_COOKIE		12

  #define RDS_INFO_FIRST			10000
  #define RDS_INFO_COUNTERS		10000

diff --git a/net/rds/message.c b/net/rds/message.c
index d874b74..e499566 100644
--- a/net/rds/message.c
+++ b/net/rds/message.c

@@ -341,12 +341,14 @@ struct rds_message *rds_message_map_pages(unsigned long *page_addrs, unsigned in
  	return rm;
  }
  
-int rds_message_copy_from_user(struct rds_message *rm, struct iov_iter *from)
+int rds_message_copy_from_user(struct rds_message *rm, struct iov_iter *from,
+			       bool zcopy)
  {
  	unsigned long to_copy, nbytes;
  	unsigned long sg_off;
  	struct scatterlist *sg;
  	int ret = 0;
+	int length = iov_iter_count(from);
  
  	rm->m_inc.i_hdr.h_len = cpu_to_be32(iov_iter_count(from));
  

@@ -356,6 +358,53 @@ int rds_message_copy_from_user(struct rds_message *rm, struct iov_iter *from)
  	sg = rm->data.op_sg;
  	sg_off = 0; /* Dear gcc, sg->page will be null from kzalloc. */
  
+	if (zcopy) {
+		int total_copied = 0;
+		struct sk_buff *skb;
+
+		skb = alloc_skb(SO_EE_ORIGIN_MAX_ZCOOKIES * sizeof(u32),
+				GFP_KERNEL);

diff --git a/net/rds/rds.h b/net/rds/rds.h
index 6e8fc4c..dfdc9b3 100644
--- a/net/rds/rds.h
+++ b/net/rds/rds.h

@@ -784,7 +784,8 @@ void rds_for_each_conn_info(struct socket *sock, unsigned int len,
  /* message.c */
  struct rds_message *rds_message_alloc(unsigned int nents, gfp_t gfp);
  struct scatterlist *rds_message_alloc_sgs(struct rds_message *rm, int nents);
-int rds_message_copy_from_user(struct rds_message *rm, struct iov_iter *from);
+int rds_message_copy_from_user(struct rds_message *rm, struct iov_iter *from,
+			       bool zcopy);
  struct rds_message *rds_message_map_pages(unsigned long *page_addrs, unsigned int total_len);
  void rds_message_populate_header(struct rds_header *hdr, __be16 sport,
  				 __be16 dport, u64 seq);

diff --git a/net/rds/send.c b/net/rds/send.c
index 5ac0925..80171cf 100644
--- a/net/rds/send.c
+++ b/net/rds/send.c

@@ -1087,8 +1112,15 @@ int rds_sendmsg(struct socket *sock, struct msghdr *msg, size_t payload_len)
  		goto out;
  	}
  
+	if (zcopy) {
+		if (rs->rs_transport->t_type != RDS_TRANS_TCP) {
+			ret = -EOPNOTSUPP;
+			goto out;
+		}
+		num_sgs = iov_iter_npages(&msg->msg_iter, INT_MAX);
+	}