SUNRPC: Handle TCP socket sends with kernel_sendpage() again
Daire Byrne reports a ~50% aggregrate throughput regression on his Linux NFS server after commit da1661b9 ("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 da1661b9 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: