Re: [PATCH RFC v2 00/12] socket sendmsg MSG_ZEROCOPY

On Tue, Feb 28, 2017 at 12:43 PM, Willem de Bruijn
[off-list ref] wrote:

quoted

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?