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

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Date:	Sun, 29 Jan 2012 22:49:40 -0800
From:	Nandita Dukkipati <nanditad@...gle.com>
To:	Neal Cardwell <ncardwell@...gle.com>
Cc:	David Miller <davem@...emloft.net>, netdev@...r.kernel.org,
	ilpo.jarvinen@...sinki.fi, 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, Jan 28, 2012 at 7:29 PM, Neal Cardwell <ncardwell@...gle.com> 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>

Acked-by: Nandita Dukkipati <nanditad@...gle.com>
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