netdev - Re: [PATCH RFC v2 00/12] socket sendmsg MSG

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Message-ID: <CALCETrUgOpcNO6L3eabAr3P8p86PGYT7BfCt9mcNTu6TNfP7Dw@mail.gmail.com>
Date:   Tue, 28 Feb 2017 13:09:44 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Michael Kerrisk <mtk.manpages@...il.com>,
        netdev <netdev@...r.kernel.org>,
        Willem de Bruijn <willemb@...gle.com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

On Tue, Feb 28, 2017 at 12:43 PM, Willem de Bruijn
<willemdebruijn.kernel@...il.com> wrote:
>
>> I can see this working if you have a special type of skb that
>> indicates that the data might be concurrently written and have all the
>> normal skb APIs (including, especially, anything that clones it) make
>> a copy first.
>
> Support for cloned skbs is required for TCP, both at tcp_transmit_skb
> and segmentation offload. Patch 4 especially adds reference counting
> of shared pages across clones and other sk_buff operations like
> pskb_expand_head. This still allows for deep copy (skb_copy_ubufs)
> on clones in specific datapaths like the above.

Does this mean that a user program that does a zerocopy send can cause
a retransmitted segment to contain different data than the original
segment?  If so, is that okay?