tcp: RX path for devmem TCP

In tcp_recvmsg_locked(), detect if the skb being received by the user
is a devmem skb. In this case - if the user provided the MSG_SOCK_DEVMEM
flag - pass it to tcp_recvmsg_devmem() for custom handling.

tcp_recvmsg_devmem() copies any data in the skb header to the linear
buffer, and returns a cmsg to the user indicating the number of bytes
returned in the linear buffer.

tcp_recvmsg_devmem() then loops over the unaccessible devmem skb frags,
and returns to the user a cmsg_devmem indicating the location of the
data in the dmabuf device memory. cmsg_devmem contains this information:

1. the offset into the dmabuf where the payload starts. 'frag_offset'.
2. the size of the frag. 'frag_size'.
3. an opaque token 'frag_token' to return to the kernel when the buffer
is to be released.

The pages awaiting freeing are stored in the newly added
sk->sk_user_frags, and each page passed to userspace is get_page()'d.
This reference is dropped once the userspace indicates that it is
done reading this page.  All pages are released when the socket is
destroyed.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Kaiyuan Zhang <kaiyuanz@google.com>
Signed-off-by: Mina Almasry <almasrymina@google.com>
Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20240910171458.219195-10-almasrymina@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index eb631e66ee03..5afe5e57c89b 100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -79,6 +79,7 @@
 #include <linux/seq_file.h>
 #include <linux/inetdevice.h>
 #include <linux/btf_ids.h>
+#include <linux/skbuff_ref.h>
 
 #include <crypto/hash.h>
 #include <linux/scatterlist.h>
@@ -2512,10 +2513,25 @@ static void tcp_md5sig_info_free_rcu(struct rcu_head *head)
 }
 #endif
 
+static void tcp_release_user_frags(struct sock *sk)
+{
+#ifdef CONFIG_PAGE_POOL
+	unsigned long index;
+	void *netmem;
+
+	xa_for_each(&sk->sk_user_frags, index, netmem)
+		WARN_ON_ONCE(!napi_pp_put_page((__force netmem_ref)netmem));
+#endif
+}
+
 void tcp_v4_destroy_sock(struct sock *sk)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
 
+	tcp_release_user_frags(sk);
+
+	xa_destroy(&sk->sk_user_frags);
+
 	trace_tcp_destroy_sock(sk);
 
 	tcp_clear_xmit_timers(sk);