[RFC PATCH v2 08/48] tcp: Support MSG_SPLICE_PAGES

David Howells
2023-03-29T14:16:59Z

Make TCP's sendmsg() support MSG_SPLICE_PAGES. This causes pages to be
spliced from the source iterator if possible (the iterator must be
ITER_BVEC and the pages must be spliceable).

This allows ->sendpage() to be replaced by something that can handle
multiple multipage folios in a single transaction.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Eric Dumazet <edumazet@google.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: netdev@vger.kernel.org
---
net/ipv4/tcp.c | 67 ++++++++++++++++++++++++++++++++++++++++++++------
1 file changed, 60 insertions(+), 7 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 288693981b00..910b327c236e 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1220,7 +1220,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
int flags, err, copied = 0;
int mss_now = 0, size_goal, copied_syn = 0;
int process_backlog = 0;
- bool zc = false;
+ int zc = 0;
long timeo;

flags = msg->msg_flags;
@@ -1231,17 +1231,22 @@ 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);
- zc = sk->sk_route_caps & NETIF_F_SG;
+ if (sk->sk_route_caps & NETIF_F_SG)
+ zc = 1;
} else if (sock_flag(sk, SOCK_ZEROCOPY)) {
uarg = msg_zerocopy_realloc(sk, size, skb_zcopy(skb));
if (!uarg) {
err = -ENOBUFS;
goto out_err;
}
- zc = sk->sk_route_caps & NETIF_F_SG;
- if (!zc)
+ if (sk->sk_route_caps & NETIF_F_SG)
+ zc = 1;
+ else
uarg_to_msgzc(uarg)->zerocopy = 0;
}
+ } else if (unlikely(msg->msg_flags & MSG_SPLICE_PAGES) && size) {
+ if (sk->sk_route_caps & NETIF_F_SG)
+ zc = 2;
}

if (unlikely(flags & MSG_FASTOPEN || inet_sk(sk)->defer_connect) &&
@@ -1304,7 +1309,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
goto do_error;

while (msg_data_left(msg)) {
- int copy = 0;
+ ssize_t copy = 0;

skb = tcp_write_queue_tail(sk);
if (skb)
@@ -1345,7 +1350,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
if (copy > msg_data_left(msg))
copy = msg_data_left(msg);

- if (!zc) {
+ if (zc == 0) {
bool merge = true;
int i = skb_shinfo(skb)->nr_frags;
struct page_frag *pfrag = sk_page_frag(sk);
@@ -1390,7 +1395,7 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
page_ref_inc(pfrag->page);
}
pfrag->offset += copy;
- } else {
+ } else if (zc == 1) {
/* First append to a fragless skb builds initial
* pure zerocopy skb
*/
@@ -1411,6 +1416,54 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
if (err < 0)
goto do_error;
copy = err;
+ } else if (zc == 2) {
+ /* Splice in data. */
+ struct page *page = NULL, **pages = &page;
+ size_t off = 0, part;
+ bool can_coalesce;
+ int i = skb_shinfo(skb)->nr_frags;
+
+ copy = iov_iter_extract_pages(&msg->msg_iter, &pages,
+ copy, 1, 0, &off);
+ if (copy <= 0) {
+ err = copy ?: -EIO;
+ goto do_error;
+ }
+
+ can_coalesce = skb_can_coalesce(skb, i, page, off);
+ if (!can_coalesce && i >= READ_ONCE(sysctl_max_skb_frags)) {
+ tcp_mark_push(tp, skb);
+ iov_iter_revert(&msg->msg_iter, copy);
+ goto new_segment;
+ }
+ if (tcp_downgrade_zcopy_pure(sk, skb)) {
+ iov_iter_revert(&msg->msg_iter, copy);
+ goto wait_for_space;
+ }
+
+ part = tcp_wmem_schedule(sk, copy);
+ iov_iter_revert(&msg->msg_iter, copy - part);
+ if (!part)
+ goto wait_for_space;
+ copy = part;
+
+ if (can_coalesce) {
+ skb_frag_size_add(&skb_shinfo(skb)->frags[i - 1], copy);
+ } else {
+ get_page(page);
+ skb_fill_page_desc_noacc(skb, i, page, off, copy);
+ }
+ page = NULL;
+
+ if (!(flags & MSG_NO_SHARED_FRAGS))
+ skb_shinfo(skb)->flags |= SKBFL_SHARED_FRAG;
+
+ skb->len += copy;
+ skb->data_len += copy;
+ skb->truesize += copy;
+ sk_wmem_queued_add(sk, copy);
+ sk_mem_charge(sk, copy);
+
}

if (!copied)