netdev - Re: [RFC PATCH v2 15/21] net/tcp: add MSG

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Message-ID: <20200727155549.gbwosugbugknsneo@bsd-mbp.dhcp.thefacebook.com>
Date:   Mon, 27 Jul 2020 08:55:49 -0700
From:   Jonathan Lemon <jonathan.lemon@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     netdev <netdev@...r.kernel.org>, kernel-team <kernel-team@...com>,
        Christoph Hellwig <hch@....de>,
        Robin Murphy <robin.murphy@....com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Miller <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Steffen Klassert <steffen.klassert@...unet.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Maxim Mikityanskiy <maximmi@...lanox.com>,
        bjorn.topel@...el.com, magnus.karlsson@...el.com,
        borisp@...lanox.com, david@...hat.com
Subject: Re: [RFC PATCH v2 15/21] net/tcp: add MSG_NETDMA flag for sendmsg()

On Mon, Jul 27, 2020 at 08:19:43AM -0700, Eric Dumazet wrote:
> On Mon, Jul 27, 2020 at 12:51 AM Jonathan Lemon
> <jonathan.lemon@...il.com> wrote:
> >
> > This flag indicates that the attached data is a zero-copy send,
> > and the pages should be retrieved from the netgpu module.  The
> > socket should should already have been attached to a netgpu queue.
> >
> > Signed-off-by: Jonathan Lemon <jonathan.lemon@...il.com>
> > ---
> >  include/linux/socket.h | 1 +
> >  net/ipv4/tcp.c         | 8 ++++++++
> >  2 files changed, 9 insertions(+)
> >
> > diff --git a/include/linux/socket.h b/include/linux/socket.h
> > index 04d2bc97f497..63816cc25dee 100644
> > --- a/include/linux/socket.h
> > +++ b/include/linux/socket.h
> > @@ -310,6 +310,7 @@ struct ucred {
> >                                           */
> >
> >  #define MSG_ZEROCOPY   0x4000000       /* Use user data in kernel path */
> > +#define MSG_NETDMA     0x8000000
> >  #define MSG_FASTOPEN   0x20000000      /* Send data in TCP SYN */
> >  #define MSG_CMSG_CLOEXEC 0x40000000    /* Set close_on_exec for file
> >                                            descriptor received through
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 261c28ccc8f6..340ce319edc9 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -1214,6 +1214,14 @@ int tcp_sendmsg_locked(struct sock *sk, struct msghdr *msg, size_t size)
> >                         uarg->zerocopy = 0;
> >         }
> >
> > +       if (flags & MSG_NETDMA && size && sock_flag(sk, SOCK_ZEROCOPY)) {
> > +               zc = sk->sk_route_caps & NETIF_F_SG;
> > +               if (!zc) {
> > +                       err = -EFAULT;
> > +                       goto out_err;
> > +               }
> > +       }
> >
> 
> Sorry, no, we can not allow adding yet another branch into TCP fast
> path for yet another variant of zero copy.

I'm not in disagreement with that statement, but the existing zerocopy
work makes some assumptions that aren't suitable.  I take it that you'd
rather have things folded together so the old/new code works together?

Allocating an extra structure for every skbuff isn't ideal in my book.


> Overall, I think your patch series desperately tries to add changes in
> TCP stack, while there is yet no proof
> that you have to use TCP transport between the peers.

The goal is having a reliable transport without resorting to RDMA.
-- 
Jonathan