Re: [PATCH 2/2] tcp: cleanup tcp_try_coalesce

Eric Dumazet
2012-05-03T04:06:31Z

On Wed, 2012-05-02 at 20:39 -0700, Alexander Duyck wrote:
> This change is mostly meant to help improve the readability of
> tcp_try_coalesce. I had a few issues since there were several points when
> the code would test for a conditional, fail, then succeed on another
> conditional take some action, and then follow a goto back into the previous
> conditional. I just tore all of that apart and made the whole thing one
> linear flow with a single goto.
>
> Also there were multiple ways of computing the delta, the one for head_frag
> made the least amount of sense to me since we were only dropping the
> sk_buff so I have updated the logic for the stolen head case so that delta
> is only truesize - sizeof(skb_buff), and for the case where we are dropping
> the head as well it is truesize - SKB_TRUESIZE(skb_end_pointer - head).
> This way we can also account for the head_frag with headlen == 0.
>
> Signed-off-by: Alexander Duyck <alexander.h.duyck@intel.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
> ---
>

Sorry I prefer you dont touch this code like this.

The truesize bits must stay as is, since it'll track drivers that lies
about truesize.

> net/ipv4/tcp_input.c | 80 +++++++++++++++++++++++++++-----------------------
> 1 files changed, 43 insertions(+), 37 deletions(-)
>
> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index c6f78e2..23bc3ff 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -4548,62 +4548,68 @@ static bool tcp_try_coalesce(struct sock *sk,
> int i, delta, len = from->len;
>
> *fragstolen = false;
> +
> if (tcp_hdr(from)->fin || skb_cloned(to))
> return false;
> +
> if (len <= skb_tailroom(to)) {
> BUG_ON(skb_copy_bits(from, 0, skb_put(to, len), len));
> -merge:
> - NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
> - TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
> - TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
> - return true;
> + goto merge;
> }
>
> if (skb_has_frag_list(to) || skb_has_frag_list(from))
> return false;
>
> - if (skb_headlen(from) == 0 &&
> - (skb_shinfo(to)->nr_frags +
> - skb_shinfo(from)->nr_frags <= MAX_SKB_FRAGS)) {
> - WARN_ON_ONCE(from->head_frag);
> - delta = from->truesize - ksize(from->head) -
> - SKB_DATA_ALIGN(sizeof(struct sk_buff));

This delta was done on purpose. We want the ksize()

> -
> - WARN_ON_ONCE(delta < len);
> -copyfrags:
> - memcpy(skb_shinfo(to)->frags + skb_shinfo(to)->nr_frags,
> - skb_shinfo(from)->frags,
> - skb_shinfo(from)->nr_frags * sizeof(skb_frag_t));
> - skb_shinfo(to)->nr_frags += skb_shinfo(from)->nr_frags;
> -
> - if (skb_cloned(from))
> - for (i = 0; i < skb_shinfo(from)->nr_frags; i++)
> - skb_frag_ref(from, i);
> - else
> - skb_shinfo(from)->nr_frags = 0;
> -
> - to->truesize += delta;
> - atomic_add(delta, &sk->sk_rmem_alloc);
> - sk_mem_charge(sk, delta);
> - to->len += len;
> - to->data_len += len;
> - goto merge;
> - }
> - if (from->head_frag && !skb_cloned(from)) {
> + if (skb_headlen(from) != 0) {
> struct page *page;
> unsigned int offset;
>
> - if (skb_shinfo(to)->nr_frags + skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
> + if (skb_shinfo(to)->nr_frags +
> + skb_shinfo(from)->nr_frags >= MAX_SKB_FRAGS)
> + return false;
> +
> + if (!from->head_frag || skb_cloned(from))
> return false;
> +
> + delta = from->truesize - sizeof(struct sk_buff);

> +
> page = virt_to_head_page(from->head);
> offset = from->data - (unsigned char *)page_address(page);
> +
> skb_fill_page_desc(to, skb_shinfo(to)->nr_frags,
> page, offset, skb_headlen(from));
> *fragstolen = true;
> - delta = len; /* we dont know real truesize... */
> - goto copyfrags;
> + } else {
> + if (skb_shinfo(to)->nr_frags +
> + skb_shinfo(from)->nr_frags > MAX_SKB_FRAGS)
> + return false;
> +
> + delta = from->truesize -
> + SKB_TRUESIZE(skb_end_pointer(from) - from->head);

No... SKB_TRUESIZE() doesnt account of power-of-two roundings in
kmalloc()

> }
> - return false;
> +
> + memcpy(skb_shinfo(to)->frags + skb_shinfo(to)->nr_frags,
> + skb_shinfo(from)->frags,
> + skb_shinfo(from)->nr_frags * sizeof(skb_frag_t));
> + skb_shinfo(to)->nr_frags += skb_shinfo(from)->nr_frags;
> +
> + if (!skb_cloned(from))
> + skb_shinfo(from)->nr_frags = 0;
> +

You break the code here... we had an else.

> + for (i = 0; i < skb_shinfo(from)->nr_frags; i++)
> + skb_frag_ref(from, i);
> +
> + to->truesize += delta;
> + atomic_add(delta, &sk->sk_rmem_alloc);
> + sk_mem_charge(sk, delta);
> + to->len += len;
> + to->data_len += len;
> +
> +merge:
> + NET_INC_STATS_BH(sock_net(sk), LINUX_MIB_TCPRCVCOALESCE);
> + TCP_SKB_CB(to)->end_seq = TCP_SKB_CB(from)->end_seq;
> + TCP_SKB_CB(to)->ack_seq = TCP_SKB_CB(from)->ack_seq;
> + return true;
> }
>
> static void kfree_skb_partial(struct sk_buff *skb, bool head_stolen)
>

Really this patch is too hard to review.

Thanks