Return-Path: Received: from mail-oi0-f43.google.com ([209.85.218.43]:35003 "EHLO mail-oi0-f43.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932072AbbGFNgj (ORCPT ); Mon, 6 Jul 2015 09:36:39 -0400 Received: by oihr66 with SMTP id r66so61777520oih.2 for ; Mon, 06 Jul 2015 06:36:38 -0700 (PDT) MIME-Version: 1.0 In-Reply-To: <20150706112647.2d4c3780@noble> References: <20150629142623.5afc0e6d@noble> <1435931379-6654-1-git-send-email-trond.myklebust@primarydata.com> <20150706112647.2d4c3780@noble> Date: Mon, 6 Jul 2015 09:36:38 -0400 Message-ID: Subject: Re: [PATCH] SUNRPC: handle -EAGAIN from socket writes better. From: Trond Myklebust To: NeilBrown Cc: Linux NFS Mailing List Content-Type: text/plain; charset=UTF-8 Sender: linux-nfs-owner@vger.kernel.org List-ID: Hi Neil, On Sun, Jul 5, 2015 at 9:26 PM, NeilBrown wrote: > On Fri, 3 Jul 2015 09:49:37 -0400 Trond Myklebust > wrote: > >> Hi Neil, >> >> There should already be a handler for ENOBUFS in call_status, but >> I can see that it has a couple of flaws. What say we try to fix that >> instead? >> >> Cheers >> Trond >> > > Hi Trond, > your patches make sense I think, but they are only part of a solution. > In the problem case the error comes from sk_stream_wait_memory, and > that returns EAGAIN, never ENOBUFS. So fixing the handling of ENOBUFS > won't be enough. > > The call path is > xs_tcp_send_request -> xs_sendpages -> xs_send_pagedata -> > (sock->ops->sendpage == tcp_sendpage) -> do_tcp_sendpage -> > sk_stream_wait_memory > > and EAGAIN travels all the way from bottom to top unmolested. > > We could possibly change sk_stream_wait_memory to return ENOBUFS if > vm_wait is != 0, but: > - that could have other consequences so needs to go through netdev > and probably isn't a quick fix > - there could be other paths that don't return ENOBUFS - it really > don't seem that ENOBUFS appears all that much in 'net/' in places > where it would need to... > > Maybe we could check and translate the error in xs_sendpages: > > diff --git a/net/sunrpc/xprtsock.c b/net/sunrpc/xprtsock.c > index 66891e32c5e3..8474d79ec2b2 100644 > --- a/net/sunrpc/xprtsock.c > +++ b/net/sunrpc/xprtsock.c > @@ -431,6 +431,14 @@ out: > if (err > 0) { > *sent_p += err; > err = 0; > + } else if (err == -EAGAIN) { > + /* Might be wrong error code. */ > + if (sock->sk->sk_write_space == xs_tcp_write_space && > + sk_stream_is_writeable(sock->sk)) > + err = -ENOBUFS; > + if (sock->sk->sk_write_space == xs_udp_write_space && > + sock_writeable(sock->sk)) > + err = -ENOBUFS; > } > return err; > } > > > Though that is a bit of a hack. If/when net-dev gets the correct error > returns, we can then remove that. > > Though I'm beginning to wonder if ENOBUFS is the correct error code > anyway. "man 2 send" suggests ENOMEM, with ENOBUFS meaning: > > The output queue for a network interface was full. This > generally indicates that the interface has stopped send- > ing, but may be caused by transient congestion. (Nor- > mally, this does not occur in Linux. Packets are just > silently dropped when a device queue overflows.) > > So I'm not sure I feel to comfortable about relying on the exact error > code. > > What do you think? The latest POSIX base specifications at http://pubs.opengroup.org/onlinepubs/9699919799/functions/sendmsg.html state that sendmsg() "may fail" if: [ENOBUFS] Insufficient resources were available in the system to perform the operation. [ENOMEM] Insufficient memory was available to fulfill the request. which implies that there is no actual standard here. I therefore agree that we should just massage those error messages into something that works for us until the networking community figures out how they want to standardise. Cheers Trond