netdev - Re: Why do we need MSG_SENDPAGE

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Date:   Thu, 04 May 2017 11:32:56 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Ilya Lesokhin <ilyal@...lanox.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        tls-fpga-sw-dev <tls-fpga-sw-dev@...lanox.com>,
        Dave Watson <davejwatson@...com>
Subject: Re: Why do we need MSG_SENDPAGE_NOTLAST?

On Thu, 2017-05-04 at 17:03 +0000, Ilya Lesokhin wrote:
> 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 answer lies to how splice() is working.

User can issue one splice without MSG_MORE semantic, right ?

Still, we want an implicit MORE behavior for all individual pages, but
the last one.


> 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?

Make sure you handle partial writes (you want to coalesce 10 pages, but
stack will only take 5 of them)