netdev - Why do we need MSG_SENDPAGE

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Date:   Thu, 4 May 2017 17:03:24 +0000
From:   Ilya Lesokhin <ilyal@...lanox.com>
To:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>
CC:     tls-fpga-sw-dev <tls-fpga-sw-dev@...lanox.com>,
        Dave Watson <davejwatson@...com>
Subject: Why do we need MSG_SENDPAGE_NOTLAST?

I don't understand the need for MSG_SENDPAGE_NOTLAST and I'm hoping someone can enlighten me.

According to commit 35f9c09 ('tcp: tcp_sendpages() should call tcp_push() once'):
"We need to call tcp_flush() at the end of the last page processed in
tcp_sendpages(), or else transmits can be deferred and future sends
stall."

I don't understand why we need to differentiate between the user setting MSG_MORE 
and splice indicating that more data is going to be sent.
if the user passed MSG_MORE and didn't push any extra data, isn't it the users fault? 
Do we need it because poorly written applications were broken when 
MSG_MORE was added to tcp_sendpage? Or is there a deeper reason?

The reason I'm asking is that we are working on a kernel TLS implementation 
and I would like to know if we can coalesce multiple tls_sendpage calls with MSG_MORE into a single
tls record or whether we must push out the record as soon as MSG_SENDPAGE_NOTLAST is cleared?

Thanks,
Ilya