netdev - Re: [PATCH net-next 4/5] tcp: implement mmap() for zero copy receive

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Message-ID: <20180420091951.713c0b95@lwn.net>
Date:   Fri, 20 Apr 2018 09:19:51 -0600
From:   Jonathan Corbet <corbet@....net>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     Eric Dumazet <edumazet@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>
Subject: Re: [PATCH net-next 4/5] tcp: implement mmap() for zero copy
 receive

On Thu, 19 Apr 2018 18:01:32 -0700
Eric Dumazet <eric.dumazet@...il.com> wrote:

> We can keep mmap() nice interface, granted we can add one hook like in following patch.
> 
> David, do you think such patch would be acceptable by lkml and mm/fs maintainers ?
> 
> Alternative would be implementing an ioctl() or getsockopt() operation,
> but it seems less natural...

So I have little standing here, but what the heck, not letting that bother
me has earned me a living for the last 20 years or so...:)

I think you should consider switching over to an interface where you
mmap() the region once, and use ioctl() to move the data into that region,
for a couple of reasons beyond the locking issues you've already found:

 - The "mmap() consumes data" semantics are a bit ... strange, IMO.
   That's not what mmap() normally does.  People expect ioctl() to do
   magic things, instead.

 - I would expect it to be a tiny bit faster, since you wouldn't be doing
   the VMA setup and teardown each time.

Thanks,

jon