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>

3 files changed, 270 insertions, 5 deletions

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index a2fac029a84a..4f77bd862e95 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -285,6 +285,8 @@
 #include <trace/events/tcp.h>
 #include <net/rps.h>
 
+#include "../core/devmem.h"
+
 /* Track pending CMSGs. */
 enum {
 	TCP_CMSG_INQ = 1,
@@ -471,6 +473,7 @@ void tcp_init_sock(struct sock *sk)
 
 	set_bit(SOCK_SUPPORT_ZC, &sk->sk_socket->flags);
 	sk_sockets_allocated_inc(sk);
+	xa_init_flags(&sk->sk_user_frags, XA_FLAGS_ALLOC1);
 }
 EXPORT_SYMBOL(tcp_init_sock);
 
@@ -2328,6 +2331,220 @@ static int tcp_inq_hint(struct sock *sk)
 	return inq;
 }
 
+/* batch __xa_alloc() calls and reduce xa_lock()/xa_unlock() overhead. */
+struct tcp_xa_pool {
+	u8		max; /* max <= MAX_SKB_FRAGS */
+	u8		idx; /* idx <= max */
+	__u32		tokens[MAX_SKB_FRAGS];
+	netmem_ref	netmems[MAX_SKB_FRAGS];
+};
+
+static void tcp_xa_pool_commit_locked(struct sock *sk, struct tcp_xa_pool *p)
+{
+	int i;
+
+	/* Commit part that has been copied to user space. */
+	for (i = 0; i < p->idx; i++)
+		__xa_cmpxchg(&sk->sk_user_frags, p->tokens[i], XA_ZERO_ENTRY,
+			     (__force void *)p->netmems[i], GFP_KERNEL);
+	/* Rollback what has been pre-allocated and is no longer needed. */
+	for (; i < p->max; i++)
+		__xa_erase(&sk->sk_user_frags, p->tokens[i]);
+
+	p->max = 0;
+	p->idx = 0;
+}
+
+static void tcp_xa_pool_commit(struct sock *sk, struct tcp_xa_pool *p)
+{
+	if (!p->max)
+		return;
+
+	xa_lock_bh(&sk->sk_user_frags);
+
+	tcp_xa_pool_commit_locked(sk, p);
+
+	xa_unlock_bh(&sk->sk_user_frags);
+}
+
+static int tcp_xa_pool_refill(struct sock *sk, struct tcp_xa_pool *p,
+			      unsigned int max_frags)
+{
+	int err, k;
+
+	if (p->idx < p->max)
+		return 0;
+
+	xa_lock_bh(&sk->sk_user_frags);
+
+	tcp_xa_pool_commit_locked(sk, p);
+
+	for (k = 0; k < max_frags; k++) {
+		err = __xa_alloc(&sk->sk_user_frags, &p->tokens[k],
+				 XA_ZERO_ENTRY, xa_limit_31b, GFP_KERNEL);
+		if (err)
+			break;
+	}
+
+	xa_unlock_bh(&sk->sk_user_frags);
+
+	p->max = k;
+	p->idx = 0;
+	return k ? 0 : err;
+}
+
+/* On error, returns the -errno. On success, returns number of bytes sent to the
+ * user. May not consume all of @remaining_len.
+ */
+static int tcp_recvmsg_dmabuf(struct sock *sk, const struct sk_buff *skb,
+			      unsigned int offset, struct msghdr *msg,
+			      int remaining_len)
+{
+	struct dmabuf_cmsg dmabuf_cmsg = { 0 };
+	struct tcp_xa_pool tcp_xa_pool;
+	unsigned int start;
+	int i, copy, n;
+	int sent = 0;
+	int err = 0;
+
+	tcp_xa_pool.max = 0;
+	tcp_xa_pool.idx = 0;
+	do {
+		start = skb_headlen(skb);
+
+		if (skb_frags_readable(skb)) {
+			err = -ENODEV;
+			goto out;
+		}
+
+		/* Copy header. */
+		copy = start - offset;
+		if (copy > 0) {
+			copy = min(copy, remaining_len);
+
+			n = copy_to_iter(skb->data + offset, copy,
+					 &msg->msg_iter);
+			if (n != copy) {
+				err = -EFAULT;
+				goto out;
+			}
+
+			offset += copy;
+			remaining_len -= copy;
+
+			/* First a dmabuf_cmsg for # bytes copied to user
+			 * buffer.
+			 */
+			memset(&dmabuf_cmsg, 0, sizeof(dmabuf_cmsg));
+			dmabuf_cmsg.frag_size = copy;
+			err = put_cmsg(msg, SOL_SOCKET, SO_DEVMEM_LINEAR,
+				       sizeof(dmabuf_cmsg), &dmabuf_cmsg);
+			if (err || msg->msg_flags & MSG_CTRUNC) {
+				msg->msg_flags &= ~MSG_CTRUNC;
+				if (!err)
+					err = -ETOOSMALL;
+				goto out;
+			}
+
+			sent += copy;
+
+			if (remaining_len == 0)
+				goto out;
+		}
+
+		/* after that, send information of dmabuf pages through a
+		 * sequence of cmsg
+		 */
+		for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
+			skb_frag_t *frag = &skb_shinfo(skb)->frags[i];
+			struct net_iov *niov;
+			u64 frag_offset;
+			int end;
+
+			/* !skb_frags_readable() should indicate that ALL the
+			 * frags in this skb are dmabuf net_iovs. We're checking
+			 * for that flag above, but also check individual frags
+			 * here. If the tcp stack is not setting
+			 * skb_frags_readable() correctly, we still don't want
+			 * to crash here.
+			 */
+			if (!skb_frag_net_iov(frag)) {
+				net_err_ratelimited("Found non-dmabuf skb with net_iov");
+				err = -ENODEV;
+				goto out;
+			}
+
+			niov = skb_frag_net_iov(frag);
+			end = start + skb_frag_size(frag);
+			copy = end - offset;
+
+			if (copy > 0) {
+				copy = min(copy, remaining_len);
+
+				frag_offset = net_iov_virtual_addr(niov) +
+					      skb_frag_off(frag) + offset -
+					      start;
+				dmabuf_cmsg.frag_offset = frag_offset;
+				dmabuf_cmsg.frag_size = copy;
+				err = tcp_xa_pool_refill(sk, &tcp_xa_pool,
+							 skb_shinfo(skb)->nr_frags - i);
+				if (err)
+					goto out;
+
+				/* Will perform the exchange later */
+				dmabuf_cmsg.frag_token = tcp_xa_pool.tokens[tcp_xa_pool.idx];
+				dmabuf_cmsg.dmabuf_id = net_iov_binding_id(niov);
+
+				offset += copy;
+				remaining_len -= copy;
+
+				err = put_cmsg(msg, SOL_SOCKET,
+					       SO_DEVMEM_DMABUF,
+					       sizeof(dmabuf_cmsg),
+					       &dmabuf_cmsg);
+				if (err || msg->msg_flags & MSG_CTRUNC) {
+					msg->msg_flags &= ~MSG_CTRUNC;
+					if (!err)
+						err = -ETOOSMALL;
+					goto out;
+				}
+
+				atomic_long_inc(&niov->pp_ref_count);
+				tcp_xa_pool.netmems[tcp_xa_pool.idx++] = skb_frag_netmem(frag);
+
+				sent += copy;
+
+				if (remaining_len == 0)
+					goto out;
+			}
+			start = end;
+		}
+
+		tcp_xa_pool_commit(sk, &tcp_xa_pool);
+		if (!remaining_len)
+			goto out;
+
+		/* if remaining_len is not satisfied yet, we need to go to the
+		 * next frag in the frag_list to satisfy remaining_len.
+		 */
+		skb = skb_shinfo(skb)->frag_list ?: skb->next;
+
+		offset = offset - start;
+	} while (skb);
+
+	if (remaining_len) {
+		err = -EFAULT;
+		goto out;
+	}
+
+out:
+	tcp_xa_pool_commit(sk, &tcp_xa_pool);
+	if (!sent)
+		sent = err;
+
+	return sent;
+}
+
 /*
  *	This routine copies from a sock struct into the user buffer.
  *
@@ -2341,6 +2558,7 @@ static int tcp_recvmsg_locked(struct sock *sk, struct msghdr *msg, size_t len,
 			      int *cmsg_flags)
 {
 	struct tcp_sock *tp = tcp_sk(sk);
+	int last_copied_dmabuf = -1; /* uninitialized */
 	int copied = 0;
 	u32 peek_seq;
 	u32 *seq;
@@ -2520,15 +2738,44 @@ found_ok_skb:
 		}
 
 		if (!(flags & MSG_TRUNC)) {
-			err = skb_copy_datagram_msg(skb, offset, msg, used);
-			if (err) {
-				/* Exception. Bailout! */
-				if (!copied)
-					copied = -EFAULT;
+			if (last_copied_dmabuf != -1 &&
+			    last_copied_dmabuf != !skb_frags_readable(skb))
 				break;
+
+			if (skb_frags_readable(skb)) {
+				err = skb_copy_datagram_msg(skb, offset, msg,
+							    used);
+				if (err) {
+					/* Exception. Bailout! */
+					if (!copied)
+						copied = -EFAULT;
+					break;
+				}
+			} else {
+				if (!(flags & MSG_SOCK_DEVMEM)) {
+					/* dmabuf skbs can only be received
+					 * with the MSG_SOCK_DEVMEM flag.
+					 */
+					if (!copied)
+						copied = -EFAULT;
+
+					break;
+				}
+
+				err = tcp_recvmsg_dmabuf(sk, skb, offset, msg,
+							 used);
+				if (err <= 0) {
+					if (!copied)
+						copied = -EFAULT;
+
+					break;
+				}
+				used = err;
 			}
 		}
 
+		last_copied_dmabuf = !skb_frags_readable(skb);
+
 		WRITE_ONCE(*seq, *seq + used);
 		copied += used;
 		len -= used;
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);
diff --git a/net/ipv4/tcp_minisocks.c b/net/ipv4/tcp_minisocks.c
index ad562272db2e..bb1fe1ba867a 100644
--- a/net/ipv4/tcp_minisocks.c
+++ b/net/ipv4/tcp_minisocks.c
@@ -628,6 +628,8 @@ struct sock *tcp_create_openreq_child(const struct sock *sk,
 
 	__TCP_INC_STATS(sock_net(sk), TCP_MIB_PASSIVEOPENS);
 
+	xa_init_flags(&newsk->sk_user_frags, XA_FLAGS_ALLOC1);
+
 	return newsk;
 }
 EXPORT_SYMBOL(tcp_create_openreq_child);