netdev - Re: [PATCH net-next 1/5] tcp: fix SO_RCVLOWAT and RCVBUF autotuning

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Date:   Fri, 20 Apr 2018 00:04:44 -0300
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David 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>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net-next 1/5] tcp: fix SO_RCVLOWAT and RCVBUF autotuning

On Fri, Apr 20, 2018 at 02:36:52AM +0000, Eric Dumazet wrote:
> On Thu, Apr 19, 2018 at 7:02 PM Marcelo Ricardo Leitner <
> marcelo.leitner@...il.com> wrote:
>
> > Hi Eric,
>
> > As val may be changed to a smaller value by the line above, shouldn't
> > it assign sk->sk_rcvlowat again?  Otherwise it may still be bigger
> > than sk_rcvbuf.
>
> > Say val = 512k, sysctl_tcp_rmem[2] = 256k
> > val <<= 1 ,  val = 1M
> > val = min() ,  val = 256k
> > val > sk_rcvbuf
> >     sk_rcvbuf = 256k , at most, which is smaller than sk_rcvlowat
>
> > Without reassigning the application has to check how big is
> > tcp_rmem[2] and be sure to not go above /2 of it to not trip on this
> > again.
>
> I am not sure about that :
>
> Reporting an error might break existing applications that were not
> expecting setsockopt()
> to return an error, even if the value was 'probably too big to be okay'

I would argue that they are already broken but...

>
>
> > Or, as you have added a return value here, it could return -EINVAL in
> > such cases. Probably better, as then the application will not get a
> > smaller buffer than wanted later.
>
> Note that maybe some applications might first set SO_RCVLOWAT, then
> SO_RCVBUF,
> we do not want to break them.

... yeah.. if they do it this way, they work today. Good point.

>
>
> My patch really covers the case were autotuning should immediately grow the
> sk_rcvbuf
> for reasonable SO_RCVLOWAT values.

That's not exactly what the comment above the function says, thus why
my comments.

Thanks.