netdev - Re: [PATCH net] tcp: minimize false-positives on TCP/GRO check

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Date:   Mon, 03 Apr 2017 18:44:56 -0700 (PDT)
From:   David Miller <davem@...emloft.net>
To:     marcelo.leitner@...il.com
Cc:     netdev@...r.kernel.org, eric.dumazet@...il.com,
        jmaxwell37@...il.com, markus@...ppelsdorf.de
Subject: Re: [PATCH net] tcp: minimize false-positives on TCP/GRO check

From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Date: Sat,  1 Apr 2017 11:00:21 -0300

> Markus Trippelsdorf reported that after commit dcb17d22e1c2 ("tcp: warn
> on bogus MSS and try to amend it") the kernel started logging the
> warning for a NIC driver that doesn't even support GRO.
> 
> It was diagnosed that it was possibly caused on connections that were
> using TCP Timestamps but some packets lacked the Timestamps option. As
> we reduce rcv_mss when timestamps are used, the lack of them would cause
> the packets to be bigger than expected, although this is a valid case.
> 
> As this warning is more as a hint, getting a clean-cut on the
> threshold is probably not worth the execution time spent on it. This
> patch thus alleviates the false-positives with 2 quick checks: by
> accounting for the entire TCP option space and also checking against the
> interface MTU if it's available.
> 
> These changes, specially the MTU one, might mask some real positives,
> though if they are really happening, it's possible that sooner or later
> it will be triggered anyway.
> 
> Reported-by: Markus Trippelsdorf <markus@...ppelsdorf.de>
> Cc: Eric Dumazet <eric.dumazet@...il.com>
> Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>

Applied, thanks Marcelo.