netdev - Re: [PATCH] tcp: fix tcp_trim_head() to adjust segment count with skb MSS

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Message-ID: <alpine.DEB.2.00.1201300903040.12992@melkinpaasi.cs.helsinki.fi>
Date:	Mon, 30 Jan 2012 09:08:51 +0200 (EET)
From:	"Ilpo Järvinen" <ilpo.jarvinen@...sinki.fi>
To:	Neal Cardwell <ncardwell@...gle.com>
cc:	David Miller <davem@...emloft.net>,
	Netdev <netdev@...r.kernel.org>,
	Nandita Dukkipati <nanditad@...gle.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Tom Herbert <therbert@...gle.com>
Subject: Re: [PATCH] tcp: fix tcp_trim_head() to adjust segment count with
 skb MSS

On Sat, 28 Jan 2012, Neal Cardwell wrote:

> This commit fixes tcp_trim_head() to recalculate the number of
> segments in the skb with the skb's existing MSS, so trimming the head
> causes the skb segment count to be monotonically non-increasing - it
> should stay the same or go down, but not increase.
> 
> Previously tcp_trim_head() used the current MSS of the connection. But
> if there was a decrease in MSS between original transmission and ACK
> (e.g. due to PMTUD), this could cause tcp_trim_head() to
> counter-intuitively increase the segment count when trimming bytes off
> the head of an skb. This violated assumptions in tcp_tso_acked() that
> tcp_trim_head() only decreases the packet count, so that packets_acked
> in tcp_tso_acked() could underflow, leading tcp_clean_rtx_queue() to
> pass u32 pkts_acked values as large as 0xffffffff to
> ca_ops->pkts_acked().
> 
> As an aside, if tcp_trim_head() had really wanted the skb to reflect
> the current MSS, it should have called tcp_set_skb_tso_segs()
> unconditionally, since a decrease in MSS would mean that a
> single-packet skb should now be sliced into multiple segments.
> 
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>
> ---
>  net/ipv4/tcp_output.c |    6 ++----
>  1 files changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 8c8de27..4ff3b6d 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -1141,11 +1141,9 @@ int tcp_trim_head(struct sock *sk, struct sk_buff *skb, u32 len)
>  	sk_mem_uncharge(sk, len);
>  	sock_set_flag(sk, SOCK_QUEUE_SHRUNK);
>  
> -	/* Any change of skb->len requires recalculation of tso
> -	 * factor and mss.
> -	 */
> +	/* Any change of skb->len requires recalculation of tso factor. */
>  	if (tcp_skb_pcount(skb) > 1)
> -		tcp_set_skb_tso_segs(sk, skb, tcp_current_mss(sk));
> +		tcp_set_skb_tso_segs(sk, skb, tcp_skb_mss(skb));
>  
>  	return 0;
>  }

Nice catch... this could solve some non-fatal counter inconsistencies too 
that have been occuring very rarely.

Acked-by: Ilpo Järvinen <ilpo.jarvinen@...sinki.fi>

-- 
 i.