netdev - Re: [PATCH net-next] tcp: correct the skip logic in tcp_sacktag

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Message-ID: <CAMaK5_jfKogtZhdtBn91W44wrsWjE09Vm=76T1fXxemiA6pSVg@mail.gmail.com>
Date: Thu, 17 Jul 2025 21:36:37 +0800
From: Xin Guo <guoxin0309@...il.com>
To: Paolo Abeni <pabeni@...hat.com>
Cc: ncardwell@...gle.com, Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, 
	edumazet@...gle.com, davem@...emloft.net, dsahern@...nel.org, kuba@...nel.org
Subject: Re: [PATCH net-next] tcp: correct the skip logic in tcp_sacktag_skip()

Hi Paolo,
Thanks for your review, let me explain in the thread first.
1)let me start from tcp_sacktag_skip, the definition as below:
static struct sk_buff *tcp_sacktag_skip(struct sk_buff *skb, struct sock *sk,
u32 skip_to_seq)
{
if (skb && after(TCP_SKB_CB(skb)->seq, skip_to_seq))
return skb;

return tcp_sacktag_bsearch(sk, skip_to_seq);
}
the input skb is a hint to avoid search the RTX tree, and the condition is:
skb->seq > skip_to_seq(so skip_to_seq cannot be included in skb),
as below:
0----------------------------------------------|------------------>+ skb->seq
0--------------------|-------------------------------------------->+ skip_to_seq

2)let me check the code snippet in tcp_sacktag_write_queue()
the code try to speed up the search by using tcp_highest_sack(),
the code is from the rtx queue is a list, but now the rtx queue is a tree.
the mean is that if the start_seq >=tcp_highest_sack_seq(), the we use
skb=tcp_highest_sack() as a hint to speed up the lookup(avoid to
lookup the tree).
so we can see that the skb->seq <=start_seq.
then if we use the skb and start_seq to call tcp_sacktag_skip(),
the tcp_sacktag_skip will go for rtx tree lookup, so
code snippet does not take effect.

static int
tcp_sacktag_write_queue(struct sock *sk, const struct sk_buff *ack_skb,
u32 prior_snd_una, struct tcp_sacktag_state *state)
{
...
while (i < used_sacks) {

if (!before(start_seq, tcp_highest_sack_seq(tp))) {
skb = tcp_highest_sack(sk);
if (!skb)
break;
}
skb = tcp_sacktag_skip(skb, sk, start_seq);

walk:
skb = tcp_sacktag_walk(skb, sk, next_dup, state,
start_seq, end_seq, dup_sack);

advance_sp:
i++;
}

3) on the other side, let me show the logic in tcp_sacktag_bsearch, the logic is
   the skb->seq should be met:
   seq=<skb->seq and seq<skb->end_seq
so the seq should be included in skb, the log is not consist with
tcp_sacktag_skip().

static struct sk_buff *tcp_sacktag_bsearch(struct sock *sk, u32 seq)
{
struct rb_node *parent, **p = &sk->tcp_rtx_queue.rb_node;
struct sk_buff *skb;

while (*p) {
parent = *p;
skb = rb_to_skb(parent);
if (before(seq, TCP_SKB_CB(skb)->seq)) {
p = &parent->rb_left;
continue;
}
//[Xin Guo] at here seq=<skb->seq
if (!before(seq, TCP_SKB_CB(skb)->end_seq)) {
p = &parent->rb_right;
continue;
}
//[Xin Guo]at here seq<skb->end_seq
return skb;
}
return NULL;
}

i hope that it is more clear now, thanks.

Regards
Guo Xin.

On Thu, Jul 17, 2025 at 4:24 PM Paolo Abeni <pabeni@...hat.com> wrote:
>
> On 7/13/25 5:22 PM, Xin Guo wrote:
> > tcp_sacktag_skip() directly return the input skb only
> > if TCP_SKB_CB(skb)->seq>skip_to_seq,
> > this is not right, and  the logic should be
> > TCP_SKB_CB(skb)->seq>=skip_to_seq,
>
> Adding Kuniyuki
>
> I'm not sure this statement is actually true. A more clear (and slightly
> more descriptive) commit message could help better understanding the
> issue. What is the bad behaviour you are observing?
>
> Ideally a packetdrill test case to demonstrate it would help
sorry that a packetdrill script cannot show the wrong behavior.

>
> > for example
> > if start_seq is equal to tcp_highest_sack_seq() ,
> > the start_seq is equal to seq of skb which is from
> > tcp_highest_sack().
> > and on the other side ,when
> > tcp_highest_sack_seq() < start_seq in
> > tcp_sacktag_write_queue(),
> > the skb is from tcp_highest_sack() will be ignored
> > in tcp_sacktag_skip(), so clean the logic also.
> >
> > Fixes: 75c119afe14f ("tcp: implement rb-tree based retransmit queue")
>
> At very least the fixes tag looks wrong, because AFAICS such change did
> not modify the behaviour tcp_sacktag_skip.
the commit 75c119afe14f ("tcp: implement rb-tree based retransmit queue")
change the tcp_sacktag_skip(), and not be changed till now.
so i think it is right.
>
> > Signed-off-by: Xin Guo <guoxin0309@...il.com>
>
> Thanks,
>
> Paolo
>