Re: [PATCH net-next 2/2] tcp: propagate GSO to the new skb built in tcp collapse

[Eric Dumazet]

On Fri, Jul 27, 2018 at 8:02 PM Yafang Shao <laoar.shao@xxxxxxxxx> wrote:
>
> Currently the collapsed SKB doesn't propagate the GSO information to the
> new SKB.
> The GSO should be propagated for better tracking, i.e. when this SKB is
> dropped we could know how many network segments are dropped.

What is "the GSO"  ?

>
> Signed-off-by: Yafang Shao <laoar.shao@xxxxxxxxx>
> ---
>  net/ipv4/tcp_input.c | 12 ++++++++++--
>  1 file changed, 10 insertions(+), 2 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 90f83eb..af52e4e 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4893,6 +4893,8 @@ void tcp_rbtree_insert(struct rb_root *root, struct sk_buff *skb)
>                 if (!nskb)
>                         break;
>
> +               skb_shinfo(nskb)->gso_size = skb_shinfo(skb)->gso_size;
> +               skb_shinfo(nskb)->gso_type = skb_shinfo(skb)->gso_type;

Why gso_size and gso_type are important ?

Where later in the stack these values are used ?

>                 memcpy(nskb->cb, skb->cb, sizeof(skb->cb));
>  #ifdef CONFIG_TLS_DEVICE
>                 nskb->decrypted = skb->decrypted;
> @@ -4906,18 +4908,24 @@ void tcp_rbtree_insert(struct rb_root *root, struct sk_buff *skb)
>
>                 /* Copy data, releasing collapsed skbs. */
>                 while (copy > 0) {
> -                       int offset = start - TCP_SKB_CB(skb)->seq;
>                         int size = TCP_SKB_CB(skb)->end_seq - start;
> +                       int offset = start - TCP_SKB_CB(skb)->seq;

>
>                         BUG_ON(offset < 0);
>                         if (size > 0) {
> -                               size = min(copy, size);
> +                               if (copy >= size)
> +                                       skb_shinfo(nskb)->gso_segs +=
> +                                               max_t(u16, 1, skb_shinfo(skb)->gso_segs);
> +                               else
> +                                       size = copy;
> +

So... what happens if copy was partial ?

Your patch does not really fix the uncertainty, it merely shifts it a bit.