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

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Date:   Fri, 20 Apr 2018 02:36:52 +0000
From:   Eric Dumazet <edumazet@...gle.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.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 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'


> 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.


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