[PATCH AUTOSEL 5.10 03/45] SUNRPC: Handle TCP socket sends with kernel_sendpage() again

Sasha Levin
2021-01-20T01:27:36Z

From: Chuck Lever <chuck.lever@oracle.com>

[ Upstream commit 4a85a6a3320b4a622315d2e0ea91a1d2b013bce4 ]

Daire Byrne reports a ~50% aggregrate throughput regression on his
Linux NFS server after commit da1661b93bf4 ("SUNRPC: Teach server to
use xprt_sock_sendmsg for socket sends"), which replaced
kernel_send_page() calls in NFSD's socket send path with calls to
sock_sendmsg() using iov_iter.

Investigation showed that tcp_sendmsg() was not using zero-copy to
send the xdr_buf's bvec pages, but instead was relying on memcpy.
This means copying every byte of a large NFS READ payload.

It looks like TLS sockets do indeed support a ->sendpage method,
so it's really not necessary to use xprt_sock_sendmsg() to support
TLS fully on the server. A mechanical reversion of da1661b93bf4 is
not possible at this point, but we can re-implement the server's
TCP socket sendmsg path using kernel_sendpage().

Reported-by: Daire Byrne <daire@dneg.com>
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=209439
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
net/sunrpc/svcsock.c | 86 +++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 85 insertions(+), 1 deletion(-)

diff --git a/net/sunrpc/svcsock.c b/net/sunrpc/svcsock.c
index c2752e2b9ce34..4404c491eb388 100644
--- a/net/sunrpc/svcsock.c
+++ b/net/sunrpc/svcsock.c
@@ -1062,6 +1062,90 @@ static int svc_tcp_recvfrom(struct svc_rqst *rqstp)
return 0; /* record not complete */
}

+static int svc_tcp_send_kvec(struct socket *sock, const struct kvec *vec,
+ int flags)
+{
+ return kernel_sendpage(sock, virt_to_page(vec->iov_base),
+ offset_in_page(vec->iov_base),
+ vec->iov_len, flags);
+}
+
+/*
+ * kernel_sendpage() is used exclusively to reduce the number of
+ * copy operations in this path. Therefore the caller must ensure
+ * that the pages backing @xdr are unchanging.
+ *
+ * In addition, the logic assumes that * .bv_len is never larger
+ * than PAGE_SIZE.
+ */
+static int svc_tcp_sendmsg(struct socket *sock, struct msghdr *msg,
+ struct xdr_buf *xdr, rpc_fraghdr marker,
+ unsigned int *sentp)
+{
+ const struct kvec *head = xdr->head;
+ const struct kvec *tail = xdr->tail;
+ struct kvec rm = {
+ .iov_base = &marker,
+ .iov_len = sizeof(marker),
+ };
+ int flags, ret;
+
+ *sentp = 0;
+ xdr_alloc_bvec(xdr, GFP_KERNEL);
+
+ msg->msg_flags = MSG_MORE;
+ ret = kernel_sendmsg(sock, msg, &rm, 1, rm.iov_len);
+ if (ret < 0)
+ return ret;
+ *sentp += ret;
+ if (ret != rm.iov_len)
+ return -EAGAIN;
+
+ flags = head->iov_len < xdr->len ? MSG_MORE | MSG_SENDPAGE_NOTLAST : 0;
+ ret = svc_tcp_send_kvec(sock, head, flags);
+ if (ret < 0)
+ return ret;
+ *sentp += ret;
+ if (ret != head->iov_len)
+ goto out;
+
+ if (xdr->page_len) {
+ unsigned int offset, len, remaining;
+ struct bio_vec *bvec;
+
+ bvec = xdr->bvec;
+ offset = xdr->page_base;
+ remaining = xdr->page_len;
+ flags = MSG_MORE | MSG_SENDPAGE_NOTLAST;
+ while (remaining > 0) {
+ if (remaining <= PAGE_SIZE && tail->iov_len == 0)
+ flags = 0;
+ len = min(remaining, bvec->bv_len);
+ ret = kernel_sendpage(sock, bvec->bv_page,
+ bvec->bv_offset + offset,
+ len, flags);
+ if (ret < 0)
+ return ret;
+ *sentp += ret;
+ if (ret != len)
+ goto out;
+ remaining -= len;
+ offset = 0;
+ bvec++;
+ }
+ }
+
+ if (tail->iov_len) {
+ ret = svc_tcp_send_kvec(sock, tail, 0);
+ if (ret < 0)
+ return ret;
+ *sentp += ret;
+ }
+
+out:
+ return 0;
+}
+
/**
* svc_tcp_sendto - Send out a reply on a TCP socket
* @rqstp: completed svc_rqst
@@ -1089,7 +1173,7 @@ static int svc_tcp_sendto(struct svc_rqst *rqstp)
mutex_lock(&xprt->xpt_mutex);
if (svc_xprt_is_dead(xprt))
goto out_notconn;
- err = xprt_sock_sendmsg(svsk->sk_sock, &msg, xdr, 0, marker, &sent);
+ err = svc_tcp_sendmsg(svsk->sk_sock, &msg, xdr, marker, &sent);
xdr_free_bvec(xdr);
trace_svcsock_tcp_send(xprt, err < 0 ? err : sent);
if (err < 0 || sent != (xdr->len + sizeof(marker)))
--
2.27.0