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    Subject[PATCHv2 net-2.6 2/4] [TCP]: Fix NewReno's fast rexmit/recovery problems with GSOed skb
    Date
    Fixes a long-standing bug which makes NewReno recovery crippled.
    With GSO the whole head skb was marked as LOST which is in
    violation of NewReno procedure that only wants to mark one packet
    and ended up breaking our TCP code by causing counter overflow
    because our code was built on top of assumption about valid
    NewReno procedure. This manifested as triggering a WARN_ON for
    the overflow in a number of places.

    It seems relatively safe alternative to just do nothing if
    tcp_fragment fails due to oom because another duplicate ACK is
    likely to be received soon and the fragmentation will be retried.

    Special thanks goes to Soeren Sonnenburg <kernel@nn7.de> who was
    lucky enough to be able to reproduce this so that the warning
    for the overflow was hit. It's not as easy task as it seems even
    if this bug happens quite often because the amount of outstanding
    data is pretty significant for the mismarkings to lead to an
    overflow.

    Because it's very late in 2.6.25-rc cycle (if this even makes in
    time), I didn't want to touch anything with SACK enabled here.
    Fragmenting might be useful for it as well but it's more or less
    a policy decision rather than mandatory fix. Thus there's no need
    to rush and we can postpone considering tcp_fragment with SACK
    for 2.6.26.

    In 2.6.24 and earlier, this very same bug existed but the effect
    is slightly different because of a small changes in the if
    conditions that fit to the patch's context. With them nothing
    got lost marker and thus no retransmissions happened.

    Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi>
    ---
    net/ipv4/tcp_input.c | 22 ++++++++++++++++++----
    1 files changed, 18 insertions(+), 4 deletions(-)

    diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
    index f82b573..f053bfe 100644
    --- a/net/ipv4/tcp_input.c
    +++ b/net/ipv4/tcp_input.c
    @@ -2138,7 +2138,9 @@ static void tcp_mark_head_lost(struct sock *sk, int packets)
    {
    struct tcp_sock *tp = tcp_sk(sk);
    struct sk_buff *skb;
    - int cnt;
    + int cnt, oldcnt;
    + int err;
    + unsigned int mss;

    BUG_TRAP(packets <= tp->packets_out);
    if (tp->lost_skb_hint) {
    @@ -2157,13 +2159,25 @@ static void tcp_mark_head_lost(struct sock *sk, int packets)
    tp->lost_skb_hint = skb;
    tp->lost_cnt_hint = cnt;

    + if (after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
    + break;
    +
    + oldcnt = cnt;
    if (tcp_is_fack(tp) || tcp_is_reno(tp) ||
    (TCP_SKB_CB(skb)->sacked & TCPCB_SACKED_ACKED))
    cnt += tcp_skb_pcount(skb);

    - if ((cnt > packets) ||
    - after(TCP_SKB_CB(skb)->end_seq, tp->high_seq))
    - break;
    + if (cnt > packets) {
    + if (tcp_is_sack(tp) || (oldcnt >= packets))
    + break;
    +
    + mss = skb_shinfo(skb)->gso_size;
    + err = tcp_fragment(sk, skb, (packets - oldcnt) * mss, mss);
    + if (err < 0)
    + break;
    + cnt = packets;
    + }
    +
    if (!(TCP_SKB_CB(skb)->sacked & (TCPCB_SACKED_ACKED|TCPCB_LOST))) {
    TCP_SKB_CB(skb)->sacked |= TCPCB_LOST;
    tp->lost_out += tcp_skb_pcount(skb);
    --
    1.5.2.2
    --
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