[RFC net-next io_uring 07/11] net/tcp: optimise tcp ubuf refcounting

[Pavel Begunkov <asml.silence@xxxxxxxxx>]

Add UARGFL_CALLER_PINNED letting protocols know that the caller holds a
reference to the ubuf_info and so it doesn't need additional refcounting
for purposes of keeping it alive. With that TCP can save a refcount
put/get pair per send when used with ->msg_ubuf.

Signed-off-by: Pavel Begunkov <asml.silence@xxxxxxxxx>
---
 include/linux/skbuff.h | 7 +++++++
 net/ipv4/tcp.c         | 9 ++++++---
 2 files changed, 13 insertions(+), 3 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 2b2e0020030b..45fe7f0648d0 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -522,6 +522,13 @@ enum {
 #define SKBFL_ALL_ZEROCOPY     (SKBFL_ZEROCOPY_FRAG | SKBFL_PURE_ZEROCOPY | \
                                 SKBFL_DONT_ORPHAN | SKBFL_MANAGED_FRAG_REFS)
 
+enum {
+       /* The caller holds a reference during the submission so the ubuf won't
+        * be freed until we return.
+        */
+       UARGFL_CALLER_PINNED = BIT(0),
+};
+
 /*
  * The callback notifies userspace to release buffers when skb DMA is done in
  * lower device, the skb last reference should be 0 when calling this.
diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 3152da8f4763..4925107de57d 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1229,7 +1229,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr 
*msg, size_t size)
 
                if (msg->msg_ubuf) {
                        uarg = msg->msg_ubuf;
-                       net_zcopy_get(uarg);
+                       if (!(uarg->flags & UARGFL_CALLER_PINNED))
+                               net_zcopy_get(uarg);
                        zc = sk->sk_route_caps & NETIF_F_SG;
                } else if (sock_flag(sk, SOCK_ZEROCOPY)) {
                        uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
@@ -1455,7 +1456,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr 
*msg, size_t size)
                tcp_push(sk, flags, mss_now, tp->nonagle, size_goal);
        }
 out_nopush:
-       net_zcopy_put(uarg);
+       if (uarg && !(uarg->flags & UARGFL_CALLER_PINNED))
+               net_zcopy_put(uarg);
        return copied + copied_syn;
 
 do_error:
@@ -1464,7 +1466,8 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr 
*msg, size_t size)
        if (copied + copied_syn)
                goto out;
 out_err:
-       net_zcopy_put_abort(uarg, true);
+       if (uarg && !(uarg->flags & UARGFL_CALLER_PINNED))
+               net_zcopy_put_abort(uarg, true);
        err = sk_stream_error(sk, flags, err);
        /* make sure we wake any epoll edge trigger waiter */
        if (unlikely(tcp_rtx_and_write_queues_empty(sk) && err == -EAGAIN)) {
-- 
2.37.0