netdev - Re: [PATCH] tcp: don't fragment SACKed skbs in tcp_mark_head

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Date:	Sat, 3 Mar 2012 10:03:56 -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: don't fragment SACKed skbs in tcp_mark_head_lost()

On Fri, Mar 2, 2012 at 11:36 PM, Neal Cardwell <ncardwell@...gle.com> wrote:
>
> In tcp_mark_head_lost() we should not attempt to fragment a SACKed skb
> to mark the first portion as lost. This is for two primary reasons:
>
> (1) tcp_shifted_skb() coalesces adjacent regions of SACKed skbs. When
> doing this, it preserves the sum of their packet counts in order to
> reflect the real-world dynamics on the wire. But given that skbs can
> have remainders that do not align to MSS boundaries, this packet count
> preservation means that for SACKed skbs there is not necessarily a
> direct linear relationship between tcp_skb_pcount(skb) and
> skb->len. Thus tcp_mark_head_lost()'s previous attempts to fragment
> off and mark as lost a prefix of length (packets - oldcnt)*mss from
> SACKed skbs were leading to occasional failures of the WARN_ON(len >
> skb->len) in tcp_fragment() (which used to be a BUG_ON(); see the
> recent "crash in tcp_fragment" thread on netdev).
>
> (2) there is no real point in fragmenting off part of a SACKed skb and
> calling tcp_skb_mark_lost() on it, since tcp_skb_mark_lost() is a NOP
> for SACKed skbs.
>
> Signed-off-by: Neal Cardwell <ncardwell@...gle.com>

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