On Mon, 6 Jul 2015 09:36:38 -0400 Trond Myklebust
<trond.myklebust@xxxxxxxxxxxxxxx> wrote:
> Hi Neil,
>
> On Sun, Jul 5, 2015 at 9:26 PM, NeilBrown <neilb@xxxxxxxx> wrote:
> > On Fri, 3 Jul 2015 09:49:37 -0400 Trond Myklebust
> > <trond.myklebust@xxxxxxxxxxxxxxx> 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.
>
Works for me - thanks.
I'll create some fault-injection so I can experiment with this code
path properly and let you know how I go - probably based on the patches
you sent.
Thanks,
NeilBrown
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