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

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Date:   Tue, 28 Feb 2017 15:22:11 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        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, 2017-02-28 at 14:52 -0800, Andy Lutomirski wrote:

> The user pages are a gift to the kernel.  The application  may  not
> modify this memory ever, otherwise the page cache and on-disk data may
> differ.
> 
> This is just not okay IMO.

TCP works just fine in this case.

TX checksum will be computed by the NIC after/while data is copied.

If really the application changes the data, that will not cause any
problems, other than user side consistency.

This is why we require a copy (for all buffers that came from zero-copy)
if network stack hits a device that can not offload TX checksum.

Even pwrite() does not guarantee consistency if multiple threads are
using it on overlapping regions.