netdev - Re: [PATCH net v4] tcp: warn on bogus MSS and try to amend it

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Message-Id: <20161206.110140.1700034264770453023.davem@davemloft.net>
Date:   Tue, 06 Dec 2016 11:01:40 -0500 (EST)
From:   David Miller <davem@...emloft.net>
To:     marcelo.leitner@...il.com
Cc:     netdev@...r.kernel.org, jmaxwell37@...il.com,
        alexandre.sidorenko@....com, kuznet@....inr.ac.ru,
        jmorris@...ei.org, yoshfuji@...ux-ipv6.org, kaber@...sh.net,
        tlfalcon@...ux.vnet.ibm.com, brking@...ux.vnet.ibm.com,
        eric.dumazet@...il.com
Subject: Re: [PATCH net v4] tcp: warn on bogus MSS and try to amend it

From: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Date: Mon,  5 Dec 2016 18:37:13 -0200

> There have been some reports lately about TCP connection stalls caused
> by NIC drivers that aren't setting gso_size on aggregated packets on rx
> path. This causes TCP to assume that the MSS is actually the size of the
> aggregated packet, which is invalid.
> 
> Although the proper fix is to be done at each driver, it's often hard
> and cumbersome for one to debug, come to such root cause and report/fix
> it.
> 
> This patch amends this situation in two ways. First, it adds a warning
> on when this situation occurs, so it gives a hint to those trying to
> debug this. It also limit the maximum probed MSS to the adverised MSS,
> as it should never be any higher than that.
> 
> The result is that the connection may not have the best performance ever
> but it shouldn't stall, and the admin will have a hint on what to look
> for.
> 
> Tested with virtio by forcing gso_size to 0.
> 
> v2: updated msg per David's suggestion
> v3: use skb_iif to find the interface and also log its name, per Eric
>     Dumazet's suggestion. As the skb may be backlogged and the interface
>     gone by then, we need to check if the number still has a meaning.
> v4: use helper tcp_gro_dev_warn() and avoid pr_warn_once inside __once, per
>     David's suggestion
> 
> Cc: Jonathan Maxwell <jmaxwell37@...il.com>
> Signed-off-by: Marcelo Ricardo Leitner <marcelo.leitner@...il.com>

Applied, thanks Marcelo.