From c0638c247f559e1a16ee79e54df14bca2cb679ea Mon Sep 17 00:00:00 2001 From: Neal Cardwell Date: Fri, 2 Mar 2012 21:36:51 +0000 Subject: [PATCH] tcp: don't fragment SACKed skbs in tcp_mark_head_lost() MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit 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 Acked-by: Ilpo Järvinen Acked-by: Yuchung Cheng Acked-by: Nandita Dukkipati Signed-off-by: David S. Miller --- net/ipv4/tcp_input.c | 1 + 1 file changed, 1 insertion(+) diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c index ee42d42b2f45..d9b83d198c3d 100644 --- a/net/ipv4/tcp_input.c +++ b/net/ipv4/tcp_input.c @@ -2569,6 +2569,7 @@ static void tcp_mark_head_lost(struct sock *sk, int packets, int mark_head) if (cnt > packets) { if ((tcp_is_sack(tp) && !tcp_is_fack(tp)) || + (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED) || (oldcnt >= packets)) break; -- 2.11.0