On Mon, 2021-02-08 at 19:48 +0000, Chuck Lever wrote:
On Feb 8, 2021, at 2:34 PM, Trond Myklebust < trondmy@hammerspace.com> wrote:
On Tue, 2021-01-19 at 20:25 -0500, Sasha Levin wrote:
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);
Thanks for your review!
I'm having trouble with this line. This looks like it is trying to push a slab page into kernel_sendpage().
The head and tail kvec's in rq_res are not kmalloc'd, they are backed by pages in rqstp->rq_pages[].
What guarantees that the nfsd thread won't call kfree() before the socket layer is done transmitting the page?
If I understand correctly what Neil told us last week, the page reference count on those pages is set up so that one of svc_xprt_release() or the network layer does the final put_page(), in a safe fashion.
Before da1661b93bf4 ("SUNRPC: Teach server to use xprt_sock_sendmsg for socket sends"), the original svc_send_common() code did this:
- /* send head */ - if (slen == xdr->head[0].iov_len) - flags = 0; - len = kernel_sendpage(sock, headpage, headoffset, - xdr->head[0].iov_len, flags); - if (len != xdr->head[0].iov_len) - goto out; - slen -= xdr->head[0].iov_len; - if (slen == 0) - goto out;
OK, so then only the argument kvec can be allocated on the slab (thanks to svc_deferred_recv)? Is that correct?